Tissue conduction communication using ramped drive signal

ABSTRACT

A device, such as an IMD, having a tissue conductance communication (TCC) transmitter controls a drive signal circuit and a polarity switching circuit by a controller of the TCC transmitter to generate an alternating current (AC) ramp on signal having a peak amplitude that is stepped up from a starting peak-to-peak amplitude to an ending peak-to-peak amplitude according to a step increment and step up interval. The TCC transmitter is further controlled to transmit the AC ramp on signal from the drive signal circuit and the polarity switching circuit via a coupling capacitor coupled to a transmitting electrode vector coupleable to the IMD. After the AC ramp on signal, the TCC transmitter transmits at least one TCC signal to a receiving device.

TECHNICAL FIELD

The disclosure relates generally to devices, systems and methods forcommunicating using tissue conduction communication.

BACKGROUND

Communication between two or more devices associated with a person,e.g., implanted within the person and/or attached to or otherwisecontacting the person, may be desirable in a number of applications,such as for monitoring or managing health of a patient. Communicationbetween these devices may, for example, enable the exchange ofinformation, coordinated monitoring of a health condition and/orcoordinated therapy to treat health conditions. Such systems, someexamples of which are described below, may communicate using tissueconduction communication (TCC). TCC uses the human body as the medium ofcommunication. TCC may sometimes be referred to as human body conduction(HBC) or intrabody communication.

A wide variety of implantable medical devices (IMDs) for delivering atherapy to or monitoring a physiological condition of a patient havebeen used clinically or proposed for clinical use in patients. Examplesinclude IMDs that deliver therapy to and/or monitor conditionsassociated with the heart, muscle, nerve, brain, stomach or othertissue. Some therapies include the delivery of electrical stimulation tosuch tissues. Some IMDs may employ electrodes for the delivery oftherapeutic electrical signals to such organs or tissues, electrodes forsensing intrinsic physiological electrical signals within the patient,which may be propagated by such organs or tissue, and/or other sensorsfor sensing physiological signals of a patient.

Implantable cardioverter defibrillators (ICDs), for example, may be usedto deliver high energy defibrillation and/or cardioversion shocks to apatient's heart when atrial or ventricular tachyarrhythmia, e.g.,tachycardia or fibrillation, is detected. An ICD may detect atachyarrhythmia based on an analysis of a cardiac electrogram sensed viaelectrodes, and may deliver anti-tachyarrhythmia shocks, e.g.,defibrillation shocks and/or cardioversion shocks, via electrodes. AnICD or an implantable cardiac pacemaker, as another example, may providecardiac pacing therapy to the heart when the natural pacemaker and/orconduction system of the heart fails to provide synchronized atrial andventricular contractions at rates and intervals sufficient to sustainhealthy patient function. ICDs and cardiac pacemakers may also provideoverdrive cardiac pacing, referred to as anti-tachycardia pacing (ATP),to suppress or convert detected tachyarrhythmias in an effort to avoidcardioversion/defibrillation shocks.

Some IMDs are coupled to one or more of the electrodes used to senseelectrical physiological signals and deliver electrical stimulation viaone or more leads. A medical electrical lead carrying sensing and/orelectrical therapy delivery electrodes allow the IMD housing to bepositioned a location spaced apart from the target site for sensingand/or stimulation delivery. For example, a subcutaneously orsub-muscularly implanted housing of an ICD or implantable cardiacpacemaker may be coupled to endocardial electrodes via one or moremedical electrical leads that extend transvenously to the patient'sheart. Other ICD systems, referred to as extracardiovascular ICDsystems, are not coupled to any transvenous leads, and instead sense anddeliver shocks via electrodes implanted away from the patient's heart,e.g., implanted subcutaneously or substernally. The extra-cardiovascularelectrodes may be provided along the housing of the subcutaneous ICDand/or coupled to the housing via one or more leads extendingsubcutaneously, submuscularly or substernally from the housing.

Leadless IMDs may also be used to deliver therapy to a patient, and/orsense physiological parameters of a patient. In some examples, aleadless IMD may include one or more electrodes on its outer housing todeliver therapeutic electrical stimulation to the patient, and/or senseintrinsic electrical signals of patient. For example, a leadlesspacemaker may be used to sense intrinsic depolarizations or otherphysiological parameters of the patient, and/or deliver therapeuticelectrical stimulation to the heart. A leadless pacemaker may bepositioned within or outside of the heart and, in some examples, may beanchored to a wall of the heart via a fixation mechanism.

In some situations, two or more IMDs are implanted within a singlepatient. It may be desirable for the two or more IMDs to be able tocommunicate with each other, e.g., to coordinate, or cooperativelyprovide, sensing for monitoring the patient and/or therapy delivery.Although some IMDs communicate with other medical devices, e.g., withexternal programming devices, using radio-frequency (RF) telemetry, TCCallows for communication between two or more IMDs by transmittingsignals between the electrodes of two IMDs via a conductive tissuepathway. Likewise, TCC may be utilized to communicate between an IMD andan external device having electrodes proximate to or in contact with theskin of the patient or between two external devices having electrodesproximate to or in contact with the skin of the patient.

SUMMARY

The techniques of this disclosure generally relate to TCC signaltransmission techniques performed by a device. The techniques of thisdisclosure are described in the context of an IMD. However, thetechniques can be utilized by any device, medical or non-medical,implanted or external, that communicates using TCC.

A TCC transmitter included in a transmitting IMD is configured togenerate at least one beacon signal during a wakeup mode of the TCCtransmitter and transmit data signals during a data transmission mode ofthe TCC transmitter. A TCC transmitter operating according to thetechniques disclosed herein is configured to transmit a ramp on signalto charge an alternating current (AC) coupling capacitor prior totransmitting a TCC signal, e.g., at the beginning of a TCC transmissionsession, to minimize interference of the TCC signal with electricalsignal sensing circuitry in the IMD system. The TCC transmitter may becontrolled to produce a ramp off signal to control discharge of thecoupling capacitor after a TCC signal, e.g., at the end of a TCCtransmission session.

In one example, the disclosure provides a device comprising a housingand a tissue conductance communication (TCC) transmitter enclosed by thehousing. The TCC transmitter includes a coupling capacitor for couplingTCC signals to a transmitting electrode vector. The TCC transmitter isconfigured to generate a TCC ramp on signal having a peak-to-peakamplitude that is stepped up from a starting peak-to-peak amplitude toan ending peak-to-peak amplitude according to a step increment and astep up interval, transmit the TCC ramp on signal via the couplingcapacitor coupled to the transmitting electrode vector, and transmit asecond TCC signal after the TCC ramp on signal.

In another example, the disclosure provides a method comprisinggenerating, with a tissue conduction communication (TCC) transmitter, aTCC ramp on signal having a peak-to-peak amplitude that is stepped upfrom a starting peak-to-peak amplitude to an ending peak-to-peakamplitude according to a step increment and a step up interval,transmitting, with the TCC transmitter, the TCC ramp on signal via acoupling capacitor coupled to a transmitting electrode vector, andtransmitting, with the TCC transmitter, a second TCC signal after theTCC ramp on signal.

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the techniques described in this disclosurewill be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram of an IMD system capable of TCC accordingto one example.

FIG. 2 is a conceptual diagram of an IMD system configured tocommunicate using TCC techniques disclosed herein according to anotherexample.

FIG. 3A is a conceptual diagram of a leadless intracardiac pacemakeraccording to one example.

FIG. 3B is a schematic diagram of circuitry that may be included in thepacemaker of FIG. 3A according to one example.

FIG. 4 illustrates a perspective view of a leadless pressure sensoraccording to one example.

FIG. 5 is a schematic diagram of an ICD capable of transmitting TCCsignals according to one example.

FIG. 6 is a conceptual diagram illustrating an example configuration ofa TCC transmitter that may be included in the ICD of FIG. 5 or in thepacemaker of FIG. 3B or pressure sensor of FIG. 4.

FIG. 7 is a conceptual diagram of a TCC transmission session that may beexecuted by the TCC transmitter of FIG. 6.

FIG. 8 is a diagram of operations performed by an IMD system during thewakeup mode of the TCC transmitter of FIG. 6.

FIG. 9 is a diagram of a TCC ramp on signal, beacon signal, and ramp offsignal according to one example.

FIG. 10 is a diagram of one example of a transmission session performedby a transmitting device of an IMD system.

FIG. 11 is a diagram of a data packet that may be transmitted during thedata transmission mode of FIG. 10 by the transmitting device accordingto one example.

FIG. 12 is a conceptual diagram of a portion of a data field that may beincluded in the data packet of FIG. 11 followed by a ramp off signal.

FIG. 13 is a flow chart of a method for transmitting TCC signals thatmay be performed by an IMD system according to one example.

FIG. 14 is a conceptual diagram of a method for transmitting TCC signalsduring multiple transmission sessions according to one example.

DETAILED DESCRIPTION

Wireless communication between two or more medical devices may bedesired for a number of reasons, including to exchange data and/or tocoordinate, or cooperatively provide, sensing of physiological signalsand/or therapy delivery. TCC signals may be wirelessly transmitted fromone IMD to one or more IMDs co-implanted within a patient and/or to anexternal medical device having skin or surface electrodes coupled to thepatient for transmitting and/or receiving TCC signals. Some IMDs andexternal medical devices may be configured to sense anelectrophysiological signal via sensing electrodes and/or monitorelectrical impedance such as transthoracic impedance signals. Examplesof electrophysiological signals include a cardiac electrical signalproduced by the patient's heart, an electromyogram signal produced byskeletal muscle tissue, and other electrophysiological signals producedby the brain, nerve or muscle tissue. Transmission of a communicationsignal through body tissue may cause interference with electrical signalsensing circuitry and/or may unintentionally cause electricalstimulation of muscle or nerves depending on the amplitude and frequencyof the transmitted signal.

An IMD or an external medical device that includes electrical signalsensing circuitry configured to receive an electrophysiological signalor monitor impedance may be a TCC transmitting device, an intended TCCreceiving device, or an unintended receiving device that is coupled toelectrodes within the tissue conduction pathway of a TCC signal beingtransmitted between two other devices. In each case, a transmitted TCCsignal may be received by sensing electrodes coupled to the transmittingor receiving IMD or external device and interfere with the sensingcircuitry. In other examples, a transmitting or receiving device may beconfigured to monitor the electrical impedance of one or more medicalelectrical leads or the tissue impedance between one or more electrodevectors coupled to the device. A TCC transmitter and transmissiontechniques are disclosed herein for enabling reliable communication ofmulti-byte streams of encoded data between medical devices whileminimizing the likelihood of a TCC signal interfering withelectrophysiological signal sensing circuitry, impedance monitoring, orother monitoring of electrical signals performed by a system.

FIG. 1 is a conceptual diagram of an IMD system 10 capable of TCCaccording to one example. FIG. 1 is a front view of a patient 12implanted with IMD system 10. IMD system 10 includes an ICD 14, anextra-cardiovascular electrical stimulation and sensing lead 16 coupledto ICD 14, and an intra-cardiac pacemaker 100. ICD 14 and pacemaker 100may be enabled to communicate via TCC for transmitting a variety of dataor commands. For example, ICD 14 and pacemaker 100 may be configured tocommunicate via TCC to confirm detected cardiac events or a detectedheart rhythm and/or coordinate delivery of cardiac pacing pulses forbradycardia pacing, ATP therapy, cardioversion/defibrillation (CV/DF)shocks, post-shock pacing, cardiac resynchronization therapy (CRT) orother electrical stimulation therapies in response to an abnormal heartrhythm being detected by one or both of the IMDs 14 and 100.

IMD system 10 senses cardiac electrical signals, such as R-wavesattendant to ventricular depolarizations and/or P-waves attendant toatrial depolarizations, for detecting abnormal heart rhythms with highsensitivity and specificity to enable IMD system 10 to deliver (orwithhold) appropriate therapies at appropriate times. Transmission ofTCC signals by an IMD, e.g., by ICD 14 or pacemaker 100, may causeinterference with the sensing circuitry of the transmitting IMD,resulting in false sensing of a cardiac event. Such false sensing ofcardiac events due to TCC interference with a cardiac event detectorincluded in electrical signal sensing circuitry may lead to withholdinga pacing pulse when a pacing pulse is actually needed or contribute tofalse detection of a tachyarrhythmia event. The TCC signal transmissiontechniques disclosed herein reduce the likelihood of a TCC signal beingfalsely detected as a cardiac event by a cardiac electrical signalsensing circuit of the transmitting device.

The TCC signal transmission techniques may also reduce the likelihoodthat another IMD implanted in patient 12 that is configured to senseelectrophysiological signals, such as R-waves and/or P-waves, falselysenses TCC signals as physiological signals. Another IMD implanted inpatient 12 may be the intended receiving device of the transmitted TCCsignals, e.g., pacemaker 100 receiving signals from ICD 14 or viceversa. In other cases, another IMD co-implanted in patient 12 may not bethe receiving device of transmitted TCC signals but may be configured tosense electrophysiological signals via electrodes coupled to theco-implanted IMD. A voltage signal may develop across sensing electrodesof the intended or unintended receiving device and interfere withelectrophysiological sensing and event detection. The TCC signaltransmission techniques of the present disclosure may reduce oreliminate the incidence of TCC signals being sensed aselectrophysiological signals or events by any other IMD implanted inpatient 12 or an external device having electrodes coupled to thepatient externally.

FIG. 1 is described in the context of an IMD system 10 including ICD 14and pacemaker 100 capable of sensing cardiac electrical signals producedby the patient's heart 8 and delivering cardioversion and/ordefibrillation (CV/DF) shocks and cardiac pacing pulses to the patient'sheart 8. In some examples, the TCC communication may be “one-way”communication, e.g., transmission only from ICD 14 to pacemaker 100 ortransmission only from pacemaker 100 to ICD 14. In other examples, theTCC communication may be “two-way” communication between ICD 14 andpacemaker 100 such that each of pacemaker 100 and ICD 14 can receive andtransmit information. It is recognized that aspects of the TCC signaltransmission techniques disclosed herein may be implemented in a varietyof IMD systems which may include an ICD, pacemaker, cardiac monitor orother sensing-only device, neurostimulator, drug delivery device orother implantable medical device(s). The TCC signal transmissiontechniques disclosed herein may be implemented in any IMD system thatrequires communication between one IMD and at least one other medicaldevice, implanted or external. Moreover, the techniques described hereinmay be utilized by two external devices that communicate using TCC. Thetechniques may also have non-medical applications as well for devicesthat are implanted and/or external and communicate using TCC.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a “can”electrode). In other instances, the housing 15 of ICD 14 may include aplurality of electrodes on an outer portion of the housing. The outerportion(s) of the housing 15 functioning as an electrode(s) may becoated with a material, such as titanium nitride for reducingpost-stimulation polarization artifact. Housing 15 may be used as anactive can electrode for use in delivering CV/DF shocks or other highvoltage pulses delivered using a high voltage therapy circuit. In otherexamples, housing 15 may be available for use in delivering relativelylower voltage cardiac pacing pulses and/or for sensing cardiacelectrical signals in combination with electrodes carried by lead 16. Inany of these examples, housing 15 may be used in a transmittingelectrode vector for transmitting TCC signals according to thetechniques disclosed herein.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,cardiac electrical signal sensing circuitry, therapy delivery circuitry,TCC transmitting and receiving circuitry, power sources and othercomponents for sensing cardiac electrical signals, detecting a heartrhythm, and controlling and delivering electrical stimulation pulses totreat an abnormal heart rhythm and for transmitting TCC signals topacemaker 100 and/or receiving TCC signals from pacemaker 100.

Lead 16 includes an elongated lead body 18 having a proximal end 27 thatincludes a lead connector (not shown) configured to be connected to ICDconnector assembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIG. 1, the distal portion 25of lead body 18 includes defibrillation electrodes 24 and 26 andpace/sense electrodes 28 and 30. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beselectively activated independently.

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to pacing and sensing electrodes 28 and 30. However,electrodes 24 and 26 and housing 15 may also be utilized to providepacing functionality, sensing functionality, and/or TCC signaltransmission and receiving in addition to or instead of high voltagestimulation therapy. In this sense, the use of the term “defibrillationelectrode” herein should not be considered as limiting the electrodes 24and 26 for use in only high voltage cardioversion/defibrillation shocktherapy applications. For example, electrodes 24 and 26 may be used in asensing vector used to sense cardiac electrical signals and detect anddiscriminate tachyarrhythmias. Electrodes 24 and 26 may be used in a TCCsignal transmitting electrode vector in combination with each other,collectively with housing 15, or individually with housing 15. In thecase of ICD 14 being configured to receive TCC signals from pacemaker100, electrodes 24, 26 and/or housing 15 may be used in a TCC receivingelectrode vector. The transmitting and receiving electrode vectors maybe the same or different vectors.

Electrodes 28 and 30 are relatively smaller surface area electrodeswhich are available for use in sensing electrode vectors for sensingcardiac electrical signals and may be used for delivering relatively lowvoltage pacing pulses in some configurations. Electrodes 28 and 30 arereferred to as pace/sense electrodes because they are generallyconfigured for use in low voltage applications, e.g., delivery ofrelatively low voltage pacing pulses and/or sensing of cardiacelectrical signals, as opposed to delivering high voltage cardioversiondefibrillation shocks. In some instances, electrodes 28 and 30 mayprovide only pacing functionality, only sensing functionality or both.Furthermore, one or both of electrodes 28 and 30 may be used in TCCsignal transmitting and/or receiving in some examples, e.g., inconjunction with electrodes 24, 26 and/or housing electrode 15.

ICD 14 may obtain cardiac electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing electrode vectors thatinclude combinations of electrodes 24, 26, 28, 30 and/or housing 15.Various sensing electrode vectors utilizing combinations of electrodes24, 26, 28, and 30 may be selected by sensing circuitry included in ICD14 for receiving a cardiac electrical signal via one or more sensingelectrode vectors.

In the example illustrated in FIG. 1, electrode 28 is located proximalto defibrillation electrode 24, and electrode 30 is located betweendefibrillation electrodes 24 and 26. Electrodes 28 and 30 may be ringelectrodes, short coil electrodes, hemispherical electrodes, or thelike. Electrodes 28 and 30 may be positioned at other locations alonglead body 18 and are not limited to the positions shown. In otherexamples, lead 16 may include none, one or more pace/sense electrodesand/or one or more defibrillation electrodes.

A TCC transmitting electrode vector may be selected from defibrillationelectrodes 24, 26, 28, 30 and housing 15 for transmitting TCC signalsproduced by a TCC transmitter included in ICD 14. Electrodes, such asdefibrillation electrodes 24 and 26 and housing 15, having a relativelylarge surface area may be used to transmit TCC signals to minimize theimpedance of the transmitting electrode vector. A low impedance of thetransmitting electrode vector maximizes the injected current signal.

The TCC transmitting electrode vector may be selected to both minimizeimpedance of the transmitting electrode vector and maximizetransimpedance from the transmitting electrode vector to the intendedreceiving electrode vector. As used herein, the term “transimpedance”refers to the voltage received at a TCC signal receiving electrodevector divided by the transmitted current (voltage out divided bycurrent in). As such, the transimpedance for a given TCC communicationelectrode vector for each of two IMDs configured to communicatebidirectionally is the same for communication in both directions for agiven set of transmitting and receiving electrode vectors. By maximizingtransimpedance, the voltage signal at the intended receiving electrodesis maximized for a given current signal injected into the tissueconductance pathway. As such, a low impedance of the transmittingelectrode vector and high transimpedance of the TCC pathway increasesthe received TCC signal strength (voltage signal) at the receivingelectrode vector.

Among the factors that may contribute to a maximized transimpedance ofthe TCC pathway are a substantially parallel electrical configuration ofthe transmitting and receiving electrode vectors, relatively widespacing of the transmitting electrodes, relatively wide spacing of thereceiving electrodes, and close proximity of the transmitting electrodevector to the receiving electrode vector. A transmitting electrodevector closer in proximity to the receiving electrode vector improvesthe strength of the TCC signal compared to a larger separation of thetransmitting and receiving electrode vectors. The optimal orientationfor the receiving electrode vector is parallel to the conductive tissuepathway of the current flow. A transmitting electrode vector that issubstantially electrically parallel to the receiving electrode vectorimproves the strength of the TCC signal compared to the receivingelectrode vector being orthogonal to the pathway of the current flowthrough the body tissue, which may result in a null signal.

A parallel electrical configuration between the transmitting andreceiving electrode vectors may coincide with physically parallelelectrode pairs. The physical electrode vectors may be viewed in somecases as the line the extends from one electrode of the vector to theother electrode of the vector to determine orientation of thetransmitting and received vectors relative to one another. In someinstances, however, physically parallel electrode pairs may not beelectrically parallel depending on the electrical conduction propertiesof the intervening tissues. For example, a body tissue having relativelylow electrical conductance, such as lung tissue, compared to othersurrounding tissues, may require a physical electrode configuration thatis not necessarily parallel in order to achieve an electricalconfiguration that is substantially parallel.

The TCC transmitting electrode vector may be selected to includeelectrodes that are not coupled to ICD sensing circuitry, e.g., acardiac event detector configured to sense R-waves and/or P-waves froman electrical signal received by a sensing electrode vector. Use of anelectrode for TCC signal transmission that is also coupled to a cardiacelectrical event detector or other electrical signal sensing circuitrymay increase interference with cardiac event detection or otherelectrical signal monitoring. The transmitting electrode pair may beselected to include at least one or both electrodes that are not coupledto the cardiac electrical event detector of ICD 14 so that TCC signalsthat are unintentionally received by the cardiac event detector arereceived via a transimpedance pathway from the transmitting electrodevector to the sensing electrode vector rather than directly through thesensing electrode impedance.

In other examples, however, the TCC transmitting electrode vector mayinclude one or more electrodes coupled to a cardiac electrical eventdetector included in ICD 14. A transmitting electrode vector may includeelectrodes coupled to the ICD sensing circuitry when the resultingtransmitting electrode vector is optimal in other ways, e.g., lowimpedance and high transimpedance. Transmission of TCC signals using oneor both electrodes included in a sensing electrode vector coupled to acardiac event detector may be selected in a trade-off for optimizingother considerations in achieving reliable TCC signal transmission andreception. TCC signal transmission techniques disclosed herein mayreduce or eliminate interference of the TCC signal transmission withcardiac event (or other electrophysiological signal) sensing as well asother sensing functions such as electrical impedance monitoring of amedical electrical lead or body tissue.

In one example, defibrillation electrode 24 may be selected incombination with housing 15 for transmitting TCC signals to pacemaker100. In other examples, TCC signals may be transmitted by ICD 14 usingdefibrillation electrode 26 and housing 15 or using two defibrillationelectrodes 24 and 26. The transmitting electrode vector impedance(delivered voltage divided by delivered current) may be up to hundredsof ohms. The transimpedance of the TCC pathway that includes atransmitting electrode vector including one defibrillation electrode 24or 26 paired with housing 15 may be less than 10 ohms and even less than1 ohm. A high transimpedance at the TCC signal transmission frequency isdesired to produce a relatively high voltage on the receiving electrodesfor a given injected current of the TCC signal.

The electrode pair selected for transmitting TCC signals may include oneor both of pace/sense electrodes 28 and 30 in some examples. Forexample, the pace/sense electrode 28 or 30 may be paired with housing15, defibrillation electrode 24 or defibrillation electrode 26 fortransmitting TCC signals. The impedance of the transmitting electrodevector may be increased due to the relatively smaller surface area ofpace/sense electrodes 28 and 30, which may have the effect of loweringthe injected current during TCC signal transmission and thereby loweringthe received voltage signal at the receiving electrode vector.

ICD 14 may be configured to select a TCC transmitting electrode vectorfrom among multiple possible vectors using electrodes 24, 26, 28, 30 andhousing 15 to achieve the best TCC signal strength at the receivingelectrodes of pacemaker 100 and/or minimize TCC signal interference withcardiac event detection, impedance monitoring, or other functionsperformed by the ICD sensing circuit and/or by a sensing circuit ofpacemaker 100. In some examples, multiple vectors may be used totransmit TCC signals to cover different angles in three-dimensionalspace to achieve at least one TCC transmitting electrode vector that issubstantially electrically parallel to the receiving electrode vector.The electrical configuration of a single transmitting vector relative tothe TCC receiving electrode vector may be time-varying due to heartmotion when the receiving electrode vector is within or coupled to thepatient's heart, as in the case of pacemaker 100.

In the example shown, lead 16 extends subcutaneously or submuscularlyover the ribcage 32 medially from the connector assembly 27 of ICD 14toward a center of the torso of patient 12, e.g., toward xiphoid process20 of patient 12. At a location near xiphoid process 20, lead 16 bendsor turns and extends superior subcutaneously or submuscularly over theribcage and/or sternum or substernally under the ribcage and/or sternum22. Although illustrated in FIG. 1 as being offset laterally from andextending substantially parallel to sternum 22, the distal portion 25 oflead 16 may be implanted at other locations, such as over sternum 22,offset to the right or left of sternum 22, angled laterally from sternum22 toward the left or the right, or the like. Alternatively, lead 16 maybe placed along other subcutaneous, submuscular or substernal paths. Thepath of extra-cardiovascular lead 16 may depend on the location of ICD14, the arrangement and position of electrodes carried by the lead body18, and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, and 30 locatedalong the distal portion 25 of the lead body 18. The elongatedelectrical conductors contained within the lead body 18 are eachelectrically coupled with respective defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30, which may be separate respectiveinsulated conductors within the lead body 18. The respective conductorselectrically couple the electrodes 24, 26, 28, and 30 to circuitry ofICD 14, such as a signal generator for therapy delivery and TCC signaltransmission and/or a sensing circuit for sensing cardiac electricalsignals and/or receiving TCC signals, via connections in the connectorassembly 17, including associated electrical feedthroughs crossinghousing 15.

The electrical conductors transmit therapy from a therapy deliverycircuit within ICD 14 to one or more of defibrillation electrodes 24 and26 and/or pace/sense electrodes 28 and 30 and transmit sensed electricalsignals from one or more of defibrillation electrodes 24 and 26 and/orpace/sense electrodes 28 and 30 to the sensing circuit within ICD 14.The electrical conductors also transmit TCC signals from a TCCtransmitter to electrodes selected for transmitting the TCC signals. Insome examples, ICD 14 may receive TCC signals from pacemaker 100 inwhich case the TCC signals are conducted from a receiving pair ofelectrodes of ICD 14 to a TCC signal receiver enclosed by housing 15.

The lead body 18 of lead 16 may be formed from a non-conductive materialand shaped to form one or more lumens within which the one or moreconductors extend. Lead body 18 may be a flexible lead body thatconforms to an implant pathway. In other examples, lead body 18 mayinclude one or more preformed curves. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the TCC transmission techniquesdisclosed herein are described in pending U.S. Publication No.2015/0306375 (Marshall, et al.) and pending U.S. Publication No.2015/0306410 (Marshall, et al.), both of which are incorporated hereinby reference in their entirety.

ICD 14 analyzes the cardiac electrical signals received from one or moresensing electrode vectors to monitor for abnormal rhythms, such asbradycardia, tachycardia or fibrillation. ICD 14 may analyze the heartrate and morphology of the cardiac electrical signals to monitor fortachyarrhythmia in accordance with any of a number of tachyarrhythmiadetection techniques. ICD 14 generates and delivers electricalstimulation therapy in response to detecting a tachyarrhythmia, e.g.,ventricular tachycardia (VT) or ventricular fibrillation (VF), using atherapy delivery electrode vector which may be selected from any of theavailable electrodes 24, 26, 28 30 and/or housing 15. ICD 14 may deliverATP in response to VT detection, and in some cases may deliver ATP priorto a CV/DF shock or during high voltage capacitor charging in an attemptto avert the need for delivering a CV/DF shock. If ATP does notsuccessfully terminate VT or when VF is detected, ICD 14 may deliver oneor more CV/DF shocks via one or both of defibrillation electrodes 24 and26 and/or housing 15. ICD 14 may generate and deliver other types ofelectrical stimulation pulses such as post-shock pacing pulses orbradycardia pacing pulses using a pacing electrode vector that includesone or more of the electrodes 24, 26, 28, and 30 and the housing 15 ofICD 14.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferiorly from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.

Pacemaker 100 is shown as a leadless intracardiac pacemaker configuredto receive TCC signals from ICD 14 via housing-based electrodes in theexamples presented herein and may be configured to transmit TCC signalsvia housing-based electrodes to ICD 14, examples of which areillustrated in FIG. 3A and the associated description. Pacemaker 100 maybe delivered transvenously and anchored by a fixation member at anintracardiac pacing and sensing site. For example, pacemaker 100 may beimplanted in an atrial or ventricular chamber of the patient's heart. Infurther examples, pacemaker 100 may be attached to an external surfaceof heart 8 (e.g., in contact with the epicardium) such that pacemaker100 is disposed outside of heart 8.

Pacemaker 100 is configured to deliver cardiac pacing pulses via a pairof housing-based electrodes and may be configured to sense cardiacelectrical signals for determining the need and timing of a deliveredpacing pulse. For example, pacemaker 100 may deliver bradycardia pacingpulses, rate responsive pacing pulses, ATP, post-shock pacing pulses,CRT pacing pulses, and/or other pacing therapies. Pacemaker 100 mayinclude a TCC receiver that receives and demodulates TCC signalstransmitted from ICD 14 and received by pacemaker 100 via housing-basedelectrodes. Pacemaker 100 may include a TCC transmitter that transmitsTCC signals to ICD 14 via the housing-based electrodes. Pacemaker 100 isdescribed in greater detail below in conjunction with FIG. 3. An exampleintracardiac pacemaker that may be included in an IMD system employingTCC is described in U.S. Pat. No. 8,744,572 (Greenhut et al.)incorporated herein by reference in its entirety.

In some examples, pacemaker 100 may be implanted in the right atrium,the right ventricle or the left ventricle of heart 8 to sense electricalactivity of heart 8 and deliver pacing therapy. In other examples,system 10 may include two or more intracardiac pacemakers 100 withindifferent chambers of heart 8 (e.g., within the right atrium, the rightventricle, and/or left ventricle). ICD 14 may be configured to transmitTCC signals to one or more pacemakers implanted within the patient'sheart 8 to coordinate electrical stimulation therapy delivery. Forexample, ICD 14 may transmit command signals to cause pacemaker 100 todeliver a cardiac pacing pulse, ATP therapy, or request confirmation ofsensed cardiac electrical events or a tachyarrhythmia detection.

An external device 40 is shown in telemetric communication with ICD 14by a wireless communication link 42 and pacemaker 100 via a wirelesscommunication link 44. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 and/or pacemaker 100 for transmitting andreceiving data via communication link 42 and 44, respectively.Communication link 42 or 44 may be established between ICD 14 orpacemaker 14, respectively, and external device 40 using a radiofrequency (RF) link such as BLUETOOTH®, Wi-Fi, or Medical ImplantCommunication Service (MICS) or other RF or communication frequencybandwidth. In some examples, ICD 14 or pacemaker 100 may communicatewith an external device 40 using TCC, e.g., using receiving surfaceelectrodes coupled to external device 40 are placed externally onpatient 12.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may be used to program cardiac eventsensing parameters (e.g., R-wave sensing parameters), cardiac rhythmdetection parameters (e.g., VT and VF detection parameters) and therapycontrol parameters used by ICD 14. Data stored or acquired by ICD 14,including physiological signals or associated data derived therefrom,results of device diagnostics, and histories of detected rhythm episodesand delivered therapies, may be retrieved from ICD 14 by external device40 following an interrogation command. External device 40 mayalternatively be embodied as a home monitor or hand-held device, such asa smart phone, tablet or other hand-held device.

In some examples, pacemaker 100 is not capable of bidirectionalcommunication with external device 40. ICD 14 may operate as a controldevice and pacemaker 100 as a responder. Pacemaker 100 may receive TCCcommunication signals from ICD 14 that include operating control dataand commands (which may be transmitted from external device 40 to ICD14) so that RF telemetry circuitry need not be included in pacemaker100. Pacemaker 100 may transmit data, such as information related todelivered pacing therapy and/or acquired cardiac electrical signals oncommand from ICD 14 via TCC transmissions, and ICD 14 may transmit datareceived from pacemaker 100 to external device 40 via RF communication.Alternatively, pacemaker 100 may periodically transmit data to ICD 14,which stores it until receiving a request from external device 40.

FIG. 2 is a conceptual diagram of an IMD system 200 configured tocommunicate using TCC transmission techniques disclosed herein accordingto another example. The IMD system 200 of FIG. 2 includes an ICD 214coupled to a patient's heart 8 via transvenous electrical leads 204,206, and 208. IMD system 200 may include a leadless pacemaker 100 and/ora leadless sensor 50. Sensor 50 is shown as a leadless pressure sensorpositioned in the pulmonary artery for monitoring pulmonary arterialpressure. Leadless pressure sensor 50, also referred to herein as“pressure sensor” 50, may be positioned at other intracardiac orarterial locations for monitoring blood pressure. In other examples, theIMD system 200 (or IMD system 10 of FIG. 1) may include other wirelesssensors performing sensing-only or monitoring-only functions configuredto send and/or receive TCC signals to/from ICD 214 (or ICD 14 of FIG. 1)and/or pacemaker 100. Other wireless sensors may include, for example,an electrogram (EGM) monitor, electrocardiogram (ECG) monitor, an oxygenmonitor, acoustical monitor, accelerometer, bioimpedance monitor, pHmonitor, temperature monitor, insulin monitor, or other sensing deviceincluding one or any combination of sensors.

ICD 214 includes a connector block 212 that may be configured to receivethe proximal ends of a right atrial (RA) lead 204, a right ventricular(RV) lead 206 and a coronary sinus (CS) lead 208, which are advancedtransvenously for positioning electrodes for sensing and stimulation inthree or all four heart chambers. RV lead 206 is positioned such thatits distal end is in the right ventricle for sensing RV cardiac signalsand delivering pacing or shocking pulses in the right ventricle. Forthese purposes, RV lead 206 is equipped with pacing and sensingelectrodes shown as a tip electrode 228 and a ring electrode 230. RVlead 206 is further shown to carry defibrillation electrodes 224 and226, which may be elongated coil electrodes used to deliver high voltageCV/DF pulses. Defibrillation electrode 224 may be referred to herein asthe “RV defibrillation electrode” or “RV coil electrode” because it maybe carried along RV lead 206 such that it is positioned substantiallywithin the right ventricle when distal pacing and sensing electrodes 228and 230 are positioned for pacing and sensing in the right ventricle.Defibrillation electrode 226 may be referred to herein as a “superiorvena cava (SVC) defibrillation electrode” or “SVC coil electrode”because it may be carried along RV lead 206 such that it is positionedat least partially along the SVC when the distal end of RV lead 206 isadvanced within the right ventricle.

Each of electrodes 224, 226, 228 and 230 are connected to a respectiveinsulated conductor extending within the body of RV lead 206. Theproximal end of the insulated conductors are coupled to correspondingconnectors carried by proximal lead connector 216, e.g., a DF-4connector, for providing electrical connection to ICD 214. It isunderstood that although ICD 214 is illustrated in FIG. 2 as amulti-chamber device coupled to RA lead 204 and CS lead 208 in additionto RV lead 206, ICD 214 may be configured as a dual-chamber devicecoupled to only two transvenous leads or a single-chamber device coupledto only one transvenous lead. For example, ICD 214 may be asingle-chamber device coupled to RV lead 206 and may be configured toperform the TCC techniques disclosed herein using electrodes 224, 226,228, and 230 and/or housing 215 in addition to receiving cardiacelectrical signals from heart 8 and delivering electrical stimulationtherapy to heart 8.

RA lead 204 is positioned such that its distal end is in the vicinity ofthe right atrium and the superior vena cava. Lead 204 is equipped withpacing and sensing electrodes 220 and 222, shown as a tip electrode 220and a ring electrode 222 spaced proximally from tip electrode 220. Theelectrodes 220 and 222 provide sensing and pacing in the right atriumand are each connected to a respective insulated conductor within thebody of RA lead 206. Each insulated conductor is coupled at its proximalend to a connector carried by proximal lead connector 210.

CS lead 208 is advanced within the vasculature of the left side of theheart via the coronary sinus (CS) and a cardiac vein (CV). CS lead 208is shown in FIG. 2 as having one or more electrodes 232, 234 that may beused in delivering pacing and/or sensing cardiac electrical signals inthe left chambers of the heart, i.e., the left ventricle and/or the leftatrium. The one or more electrodes 232, 234 of CS lead 208 are coupledto respective insulated conductors within the body of CS lead 208, whichprovide connection to the proximal lead connector 218.

Any of electrodes 220, 222, 224, 226, 228, 230, 232, 234 may be selectedby ICD 214 in a TCC electrode vector for transmitting and/or receivingTCC signals. In some examples, housing 215 is selected in a TCCtransmitting electrode vector along with a lead-based defibrillationelectrode, e.g., RV coil electrode 224 or SVC coil electrode 226, toprovide a low impedance and high transimpedance TCC transmittingelectrode vector. In other examples, TCC transmission is performed usingthe RV coil electrode 224 and the SVC coil electrode 226. In still otherexamples, an electrode 232 or 234 carried by the CS lead 208 may beselected in combination with housing 215, RV coil electrode 224, or SVCcoil electrode 226. It is recognized that numerous TCC transmittingelectrode vectors may be available using the various electrodes carriedby one or more of leads 204, 206 and 208 coupled to ICD 214. In someexamples, multiple vectors may be selected to promote transmission via avector that is substantially parallel to the housing-based electrodes ofpacemaker 100 or to receiving electrodes of leadless pressure sensor 50for transmitting signals to the respective pacemaker 100 or pressuresensor 50.

Housing 215 encloses internal circuitry generally corresponding to thevarious circuits and components described in conjunction with FIG. 5below, for sensing cardiac signals from heart 8, detecting arrhythmias,controlling therapy delivery and performing TCC with pacemaker 100and/or pressure sensor 50 using the techniques disclosed herein. It isrecognized that these TCC transmission techniques may be practiced inconjunction with alternative lead and electrode configurations otherthan those depicted in the examples of FIG. 1 and FIG. 2.

Pressure sensor 50 may be implanted in the pulmonary artery of thepatient for monitoring the pulmonary arterial pressure as an indicationof the hemodynamic status of the patient 12. One example of pressuresensor 50 is described below in conjunction with FIG. 4. Pressure sensor50 may be configured to receive pressure signals via a pressure sensorand receive TCC signals via a TCC receiver coupled to electrodes carriedby pressure sensor 50.

In the examples of FIGS. 1 and 2, two or more IMDs may be co-implantedin a patient and communicate via TCC to enable a system level offunctionality such as sharing the detection of arrhythmias betweendevices, synchronized timing of anti-tachyarrhythmia shocks, ATP, and/orpost-shock pacing, optimization of the resources (e.g., battery capacityor processing power) available to each device, or sharing orcoordination of physiological signal acquisition. In some examples,communication between the ICD 14 or ICD 214 and pacemaker 100 may beused to initiate therapy and/or confirm that therapy should bedelivered. Communication between ICD 14 or ICD 214 and pressure sensor50 may be used to initiate pressure signal acquisition and/or retrievalof pressure signal data from pressure sensor 50. One approach is for ICD14 or ICD 214 to function as a control device and pacemaker 100 and/orsensor 50 to function as responders. For instance, a TCC signal from ICD14 or 214 may cause pacemaker 100 to deliver a cardiac pacing pulse ortherapy.

In another example, ICD 214 may transmit a TCC command signal topressure sensor 50 for causing pressure sensor 50 to begin acquiring apressure signal. Pressure sensor 50 may be configured to transmitpressure signal data via TCC to ICD 214 or to external device 40 (shownin FIG. 1). ICD 214 may transmit a TCC command to pressure sensor 50 tocause pressure sensor 50 to transmit a pressure signal in real time,transmit a pressure signal previously acquired and stored by pressuresensor 50, or transmit pressure data derived from a pressure signalreceived by pressure sensor 50. In other examples, pressure sensor 50may be configured to transmit pressure signal data via RF telemetry toICD 214 and/or to an external device, such as device 40 shown in FIG. 1in response to a TCC command signal received from ICD 214.

During TCC signal transmission, current is driven through the patient'sbody tissue between two or more electrodes of the transmitting IMD(e.g., ICD 14 or 214). The current spreads through the patient's body,e.g., through the thorax, producing a potential field. The receiving IMD(e.g., pacemaker 100 or sensor 50 or other implanted or external device)may detect the TCC signal by measuring the potential difference betweentwo of its electrodes, e.g., two housing-based electrodes of pacemaker100 or sensor 50. Optimally, the receiving electrodes are parallel tothe tissue conduction pathway of the injected current to maximize thepotential difference developed on the receiving electrode vector. Thecurrent injected to transmit the TCC signal is of sufficient amplitudeto produce a voltage potential that can be detected by an intendedreceiving IMD but should at the same time not capture excitable bodytissue, e.g., causing unintended stimulation of nerve or muscle tissue,possibly leading to muscle contraction, pain or even cardiac capture.Any unintended stimulation of nerve or muscle tissue also likelyincreases noise received on the sensing electrodes of a device of system10 or 200.

In some cases, a co-implanted IMD may be an unintended receiver of theTCC signal. If a co-implanted IMD includes electrodes or is coupled toelectrodes for receiving electrical signals, but is not the intendedreceiver of a TCC signal, a voltage potential may develop across theelectrodes of the unintended receiver leading to interference with thenormal signal detection functions of the unintended receiver. Forexample, in system 200, ICD 214 and pressure sensor 50 may be configuredto communicate using TCC. Pacemaker 100 may be co-implanted with ICD 214and pressure sensor 50 but not configured to send or receive TCCsignals. A TCC signal transmitted by ICD 214 to pressure sensor 50 mayresult in voltage developed across the housing-based electrodes ofpacemaker 100. Pacemaker 100 may be an unintended receiver of thetransmitted TCC signal. The voltage developed across the housing-basedelectrodes of pacemaker 100 may interfere with a cardiac event detectorincluded in pacemaker 100. In other examples, a subcutaneous cardiacelectrical signal monitor having housing-based electrodes for monitoringa subcutaneously-acquired electrocardiogram (ECG) signal, such as theREVEAL LINQ™ Insertable Cardiac Monitor (available from Medtronic, Inc.,Minneapolis, Minn., USA) may be implanted in a patient having two otherIMDs configured to communicate via TCC, such as ICD 214 and pressuresensor 50. The cardiac electrical signal monitor may be an unintendedreceiver of TCC signals transmitted between ICD 214 and pressure sensor50. The methods disclosed herein for transmitting TCC signals mayeliminate or minimize interference of TCC signals with electrical signalsensing circuitry of other IMDs or external devices in or on thepatient, which may be intended or unintended receivers.

While particular IMD systems 10 and 200, including an ICD 14 or ICD 214,respectively, pacemaker 100 and/or pressure sensor 50 are shown in theillustrative examples of FIGS. 1 and 2, methodologies described hereinfor TCC transmission may be used with other IMD systems including othertypes and locations of IMDs as well as other lead and electrodearrangements. For example, an implantable cardiac monitor, such as theREVEAL LINQ™ Insertable Cardiac Monitor, may be utilized as a relaydevice for leadless pacemaker 100 and/or pressure sensor 50 by receivingdata from those devices via TCC and transmitting that data to anexternal device 40 via RF communication, such as BLUETOOTH™communication. Generally, this disclosure describes various techniquesfor transmitting TCC signals by an IMD that includes sensing circuitryfor sensing a cardiac electrical signal. The TCC signal transmissiontechniques reduce the likelihood that a TCC signal is oversensed as aphysiological event by the sensing circuitry of the transmitting device.The TCC transmission techniques may also reduce the likelihood of TCCsignal oversensing by sensing circuitry included in another IMDco-implanted with the transmitting device. Another IMD co-implanted withthe transmitting device may be the intended receiving device of the TCCsignal transmission or another IMD that is not the targeted recipientand may not even be configured to receive and detect TCC communicationsignals.

FIG. 3A is a conceptual diagram of pacemaker 100 according to oneexample. As shown in FIG. 3A, pacemaker 100 may be a leadless pacemakerincluding a housing 150, housing end cap 158, distal electrode 160,proximal electrode 152, fixation member 162, and a delivery toolinterface member 154. Housing 150, sealed with end cap 158, encloses andprotects the various electrical components within pacemaker 100.Pacemaker 100 is shown including two electrodes 152 and 160 but mayinclude two or more electrodes for delivering cardiac electricalstimulation pulses (such as pacing pulses or ATP), sensing cardiacelectrical signals for detecting cardiac electrical events, and forreceiving and/or transmitting TCC signals.

Electrodes 152 and 160 are carried on the housing 150 and housing endcap 158. In this manner, electrodes 152 and 160 may be consideredhousing-based electrodes. In other examples, one or more electrodes maybe coupled to circuitry enclosed by housing 150 via an electrodeextension extending away from housing 150. In the example of FIG. 3A,electrode 160 is disposed on the exterior surface of end cap 158.Electrode 160 may be a tip electrode positioned to contact cardiactissue upon implantation and fixation at a pacing site by fixationmember 162. Electrode 152 may be a ring or cylindrical electrodedisposed along the exterior surface of housing 150. Both housing 150 andhousing end cap 158 may be electrically insulating. In some examples,housing 150 is an electrically conductive material, e.g., a titaniumalloy or other biocompatible metal or metal alloy. Portions of housing150 may be coated with a non-conductive material, e.g., parylene,polyurethane, silicone or other biocompatible polymer, to insulateportions of housing 150 not functioning as electrode 152.

Electrodes 160 and 152 may be used as a cathode and anode pair forcardiac pacing therapy and receiving and/or transmitting TCC signals. Inaddition, electrodes 152 and 160 may be used to detect intrinsicelectrical signals from the patient's heart 8. In other examples,pacemaker 100 may include three or more electrodes, where any two ormore of the electrodes may be selected to form a vector for delivery ofelectrical stimulation therapy, detecting intrinsic cardiac electricalsignals from the patient's heart 8, transmitting TCC signals, andreceiving TCC signals. In some examples in which pacemaker 100 includesthree or more electrodes, two or more of the electrodes may be selected,e.g., via switches, to form a vector for TCC. Pacemaker 100 may usemultiple vectors for TCC transmission or receiving, for example, topromote a substantially parallel electrical configuration with a TCCtransmitting electrode vector of ICD 14 or ICD 214, which may increasethe transimpedance and increase the received voltage signal.

Fixation member 162 may include multiple tines of a shape memorymaterial that retains a preformed curved shape as shown. Duringimplantation, fixation member 162 may be flexed forward to pierce tissueand elastically flex back towards housing 150 to regain their pre-formedcurved shape. In this manner, fixation member 162 may be embedded withincardiac tissue at the implant site. In other examples, fixation member162 may include helical fixation tines, barbs, hooks or other fixationfeatures.

Delivery tool interface member 154 may be provided for engaging with adelivery tool used to advance pacemaker 100 to an implant site. Adelivery tool may be removably coupled to delivery tool interface member154 for retrieving pacemaker 100 back into a delivery tool if removal orrepositioning of pacemaker 100 is required.

FIG. 3B is a schematic diagram of circuitry that may be enclosed bypacemaker housing 150 according to one example. Pacemaker housing 150may enclose a control circuit 170, memory 172, pulse generator 176,sensing circuit 174, and a power source 178. Control circuit 170 mayinclude a microprocessor and/or other control circuitry for controllingthe functions attributed to pacemaker 100 herein, such as controllingpulse generator 176 to deliver signals via electrodes 152 and 160 andcontrolling sensing circuit 174 to detect signals from electricalsignals received via electrodes 152 and 160. Power source 178 mayinclude one or more rechargeable or non-rechargeable batteries forproviding power to control circuit 170, memory 172, pulse generator 176and sensing circuit 174 as needed. Control circuit 170 may executeinstructions stored in memory 172 and may control pulse generator 176and sensing circuit 174 according to control parameters stored in memory172, such as various timing intervals, pacing pulse parameters andcardiac event sensing parameters.

Pulse generator 176 generates therapeutic pacing pulses delivered viaelectrodes 152 and 160 under the control of timing circuitry included incontrol circuit 170. Pulse generator 176 may include charging circuitry,one or more charge storage devices such as one or more capacitors, andswitching circuitry that couples the charge storage device(s) to anoutput capacitor coupled to electrodes 160 and 152 to discharge thecharge storage devices via electrodes 160 and 152. In some examples,pulse generator includes a TCC transmitter (standalone or as part of atransceiver), such as the transmitter described below in conjunctionwith FIG. 6, for generating TCC signals transmitted via electrodes 160and 152. Power source 178 provides power to the charging circuit ofpulse generator 176 and the TCC transmitter when present.

Pacemaker 100 may be configured for sensing cardiac electrical signals,e.g., R-waves or P-waves, and include a cardiac event detector 173.Intrinsic cardiac electrical events may be detected from an electricalsignal produced by the heart and received via electrodes 152 and 160.Cardiac event detector 173 may include filters, amplifiers, ananalog-to-digital converter, rectifier, comparator, sense amplifier orother circuitry for detecting cardiac events from a cardiac electricalsignal received via electrodes 152 and 160. Under the control of controlcircuit 170, cardiac event detector 173 may apply various blankingand/or refractory periods to circuitry included in event detector 173and an auto-adjusting cardiac event detection threshold amplitude, e.g.,an R-wave detection threshold amplitude or a P-wave detection thresholdamplitude, to the electrical signal received via electrodes 152 and 160.

Sensing circuit 174 may further include a TCC signal detector 175 fordetecting a TCC signal from ICD 14 (or ICD 214). A voltage potentialdevelops across electrodes 152 and 160 in response to current conductedvia a tissue pathway during TCC signal transmission from ICD 14 or ICD214. The voltage signal may be received and demodulated by TCC signaldetector 175 and decoded by control circuit 170. TCC signal detector 175may include amplifiers, filters, analog-to-digital converters,rectifiers, comparators, counters, a phase locked loop and/or othercircuitry configured to detect a wakeup beacon signal from atransmitting device and detect and demodulate the modulated carriersignal transmitted in data packets including encoded data. For example,TCC signal detector 175 of pacemaker 100 (and other TCC signal detectorsreferred to herein) may include a pre-amplifier and a high-Q filtertuned to the carrier frequency of a carrier signal that is used totransmit beacon signals and data signals during a TCC transmissionsession. The filter may be followed by another amplifier and ademodulator that converts the received signals to a binary signalrepresenting coded data.

The circuitry of TCC signal detector 175 may include circuitry sharedwith cardiac event detector 173 in some examples. The filters includedin TCC signal detector 175 and cardiac event detector 173, however, areexpected to operate at different passbands, for example, for detectingdifferent signal frequencies. The TCC signals may be transmitted with acarrier frequency in the range of 33 to 250 kHz, in the range of 60 to200 kHz, or at 100 kHz as examples. Cardiac electrical signals generatedby heart 8 are generally less than 100 Hz. The TCC signal transmissiontechniques disclosed herein may reduce or eliminate oversensing of areceived TCC signal, e.g., transmitted from ICD 14 or ICD 214, as acardiac electrical event by cardiac event detector 173. In examples thatinclude a TCC transmitter in pacemaker 100, the TCC signal transmissiontechniques disclosed herein may reduce or prevent oversensing of a TCCsignal produced by the TCC transmitter and transmitted via electrodes152 and 160 from being detected as a cardiac event by cardiac eventdetector 173. In some instances, the TCC transmitter or transceiver mayinclude circuitry shared with pulse generator 176, such that the TCCsignals are transmitted using the pacing circuitry of pacemaker 100and/or transmitted as sub-threshold pacing pulses or pacing pulses thatoccur during the refractory period of the heart.

In other examples, pacemaker 100 may include fewer or more componentsthan the circuits and components shown in FIG. 3B. For instance,pacemaker 100 may include other physiological sensors and/or an RFtelemetry circuit for communication with external device 40 instead ofor in addition to TCC signal detector 175 and a TCC transmitter (ifincluded).

FIG. 4 illustrates a perspective view of leadless pressure sensor 50according to one example. Leadless pressure sensor 50 may generallycorrespond to the IMD disclosed in U.S. Pat. Publication No.2012/0323099 A1 (Mothilal, et al.), incorporated herein by reference inits entirety. As shown in FIG. 4, pressure sensor 50 includes anelongated housing 250 having a pressure sensitive diaphragm or window252 that exposes a pressure sensitive element within housing 250 to thesurrounding pressure. Electrodes 260 and 262 may be secured to oppositeends of housing 250 and may be electrically insulated from housing 250to form an electrode pair for receiving TCC signals. Electrodes 260 and262 may be coupled to a TCC signal detector (corresponding to the TCCsignal detector 175 described above) enclosed by housing 250. The TCCsignal detector is configured to detect and demodulate TCC signalsreceived from ICD 14 or ICD 214.

Housing 250 may enclose a battery, a pressure sensing circuit, a TCCsignal detector, control circuitry, and memory for storing pressuresignal data. In some examples, the pressure sensing circuit includes anair gap capacitive element and associated circuitry, which may includetemperature compensation circuitry, for producing a signal correlated topressure along window 252. The pressure sensing circuit and window 252may correspond to a pressure sensor module as generally disclosed inU.S. Pat. No. 8,720,276 (Kuhn, et al.), incorporated herein by referencein its entirety. The pressure sensing circuit may include a microelectro-mechanical system (MEMS) device in some examples. A fixationmember 270 extends from housing 250 and may include a self-expandingstent or one or more self-expanding loops 272 that stabilize theposition of pressure sensor 50 along an artery, such as the pulmonaryartery, by gently pressing against the interior walls of the artery.When deployed in an arterial location, pressure sensor 50 produces andstores pressure signals correlated to arterial blood pressure.

In some examples, pressure sensor 50 includes a TCC transmitter ortransceiver, such as the transmitter shown in FIG. 6, for transmittingTCC signals to another medical device, such as ICD 14 or ICD 214,pacemaker 100 or external device 40. Pressure sensor 50 may transmit apressure signal, data extracted from a pressure signal or othercommunication data in a TCC signal via electrodes 260 and 262. Forinstance, pressure sensor 50 may include a TCC transmitter ortransceiver for at least producing acknowledgment and/or confirmationsignals transmitted back to a transmitting device, e.g., ICD 14 or ICD214, in response to receiving a TCC signal to confirm detection of abeacon signal and/or reception of transmitted data packets.

FIG. 5 is a schematic diagram of an ICD capable of transmitting TCCsignals according to one example. For illustrative purposes, ICD 14 ofFIG. 1 is depicted in FIG. 5 coupled to electrodes 24, 26, 28, and 30,with housing 15 represented schematically as an electrode. It is to beunderstood, however, that the circuitry and components shown in FIG. 5may generally correspond to circuitry included in ICD 214 of FIG. 2 andadapted accordingly for single, dual, or multi-chamber cardiac signalsensing and therapy delivery functions using electrodes carried bytransvenous leads. For instance, in the example of the multi-chamber ICD214 of FIG. 2, signal generator 84 may include multiple therapy deliveryoutput channels and sensing circuit 86 may include multiple sensingchannels each selectively coupled to respective electrodes of RA lead204, RV lead 206 and CS lead 208, corresponding to each cardiac chamber,e.g., the right atrium, the right ventricle, and the left ventricle.

The ICD circuitry may include a control circuit 80, memory 82, signalgenerator 84, sensing circuit 86, and RF telemetry circuit 88. A powersource 89 provides power to the circuitry of the ICD, including each ofthe circuits 80, 82, 84, 86, and 88 as needed. Power source 89 mayinclude one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 89 and each of the other circuits 80, 82, 84, 86 and 88 areto be understood from the general block diagram of FIG. 5, but are notshown for the sake of clarity. For example, power source 89 may becoupled to charging circuits included in signal generator 84 forcharging capacitors or other charge storage devices included in therapycircuit 85 for producing electrical stimulation pulses such as CV/DFshock pulses and pacing pulses. Power source 89 is coupled to TCCtransmitter 90 for providing power for generating TCC signals. Powersource 89 provides power to processors and other components of controlcircuit 80, memory 82, amplifiers, analog-to-digital converters andother components of sensing circuit 86, and a transceiver of RFtelemetry circuit 88, as examples.

Memory 82 may store computer-readable instructions that, when executedby a processor included in control circuit 80, cause ICD 14 to performvarious functions attributed to ICD 14 (e.g., detection of arrhythmias,communication with pacemaker 100 or pressure sensor 50, and/or deliveryof electrical stimulation therapy). Memory 82 may include any volatile,non-volatile, magnetic, optical, or electrical media, such as a randomaccess memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or anyother digital or analog media.

Control circuit 80 communicates with signal generator 84 and sensingcircuit 86 for sensing cardiac electrical activity, detecting cardiacrhythms, and controlling delivery of cardiac electrical stimulationtherapies in response to sensed cardiac signals. The functional blocksshown in FIG. 5 represent functionality included in ICD 14 (or ICD 214)and may include any discrete and/or integrated electronic circuitcomponents that implement analog and/or digital circuits capable ofproducing the functions attributed to ICD 14 herein. Providing software,hardware, and/or firmware to accomplish the described functionality inthe context of any modern IMD system, given the disclosure herein, iswithin the abilities of one of skill in the art.

Sensing circuit 86 may be selectively coupled to electrodes 24, 26, 28,30 and/or housing 15 in order to monitor electrical activity of thepatient's heart 8. Sensing module 86 may include switching circuitry forselecting which of electrodes 24, 26, 28, 30 and housing 15 are coupledto sense amplifiers or other cardiac event detection circuitry includedin cardiac event detector 85. Switching circuitry may include a switcharray, switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple sense amplifiers to selected electrodes.The cardiac event detector 85 within sensing circuit 86 may include oneor more sense amplifiers, filters, rectifiers, threshold detectors,comparators, analog-to-digital converters (ADCs), or other analog ordigital components configured to detect cardiac electrical events from acardiac electrical signal received from heart 8.

In some examples, sensing circuit 86 includes multiple sensing channelsfor acquiring cardiac electrical signals from multiple sensing vectorsselected from electrodes 24, 25, 28, 30 and housing 15. Each sensingchannel may be configured to amplify, filter, digitize and rectify thecardiac electrical signal received from selected electrodes coupled tothe respective sensing channel to improve the signal quality for sensingcardiac events, e.g., P-waves attendant to atrial depolarizations and/orR-waves attendant to ventricular depolarizations. For example, eachsensing channel in sensing circuit 86 may include an input or pre-filterand amplifier for receiving a cardiac electrical signal developed acrossa selected sensing electrode vector, an analog-to-digital converter, apost-amplifier and filter, and a rectifier to produce a filtered,digitized, rectified and amplified cardiac electrical signal that ispassed to a cardiac event detector included in sensing circuit 86. Thecardiac event detector 85 may include a sense amplifier, comparator orother circuitry for comparing the rectified cardiac electrical signal toa cardiac event sensing threshold, such as an R-wave sensing thresholdamplitude, which may be an auto-adjusting threshold. Sensing circuit 86may produce a sensed cardiac event signal in response to a sensingthreshold crossing. The sensed cardiac events, e.g., R-waves and/orP-waves, are used for detecting cardiac rhythms and determining a needfor therapy by control circuit 80. ICD 214 of FIG. 2 may include asensing circuit having a separate atrial sensing channel for sensingP-waves using atrial electrodes and a ventricular sensing channel forsensing R-waves using ventricular electrodes.

Control circuit 80 may include interval counters, which may be resetupon receipt of a cardiac sensed event signal from sensing circuit 86.The value of the count present in an interval counter when reset by asensed R-wave or P-wave may be used by control circuit 80 to measure thedurations of R-R intervals, P-P intervals, P-R intervals and R-Pintervals, which are measurements that may be stored in memory 82.Control circuit 80 may use the count in the interval counters to detecta tachyarrhythmia event, such as atrial fibrillation (AF), atrialtachycardia (AT), VF or VT. These intervals may also be used to detectthe overall heart rate, ventricular contraction rate, and heart ratevariability.

Signal generator 84 includes a therapy circuit 92 and a TCC transmitter90. The therapy circuit 92 is configured to generate cardiac electricalstimulation pulses, e.g., CV/DF shock pulses and cardiac pacing pulsesfor delivery to heart 8 via electrodes carried by lead 16 (and in somecases housing 15). Signal generator 84 may include one or more energystorage elements, such as one or more capacitors, configured to storethe energy required for a therapeutic CV/DF shock or pacing pulse. Inresponse to detecting a shockable tachyarrhythmia, control circuit 80controls therapy circuit 83 to charge the energy storage element(s) toprepare for delivering a CV/DF shock. Therapy circuit 83 may includeother circuitry, such as charging circuitry, which may include atransformer and/or a charge pump, to charge the energy storage element,and switches to couple the energy storage element to an output capacitorto discharge and deliver the CV/DF shock and change the polarity of theshock to provide a bi-phasic or multi-phasic shock. Therapy circuit 83may include a variety of voltage level-shifting circuitry, switches,transistors, diodes, or other circuitry. Therapy circuit 83 may includeswitching circuitry for selecting a shock delivery vector and deliversthe shock therapy to the patient's heart 8 via the shock deliveryvector, e.g., two or more electrodes such as defibrillation electrode 24or 26 and housing 15.

In some examples, therapy circuit 83 may include both a low voltagetherapy circuit for generating and delivering relatively low voltagetherapy pulses, such as pacing pulses, and a high voltage therapycircuit for generating and delivering CV/DF shocks. Low voltage pacingpulses may be delivered via a pacing electrode vector selected fromelectrodes 24, 26, 28, 30 and housing 15. Pacing pulses may be deliveredwhen a pacing escape interval set by a pace timing circuit of controlcircuit 80 times out without a sensed cardiac event causing the escapeinterval to be reset. The pace timing circuit may set various escapeintervals for timing pacing pulses, e.g., to provide bradycardia pacingor post-shock pacing, or in response to detecting a tachyarrhythmia bydelivering ATP. In some examples, pacemaker 100 is provided fordelivering at least some low voltage pacing therapies, e.g., whensignaled to do so by a TCC signal transmitted from ICD 14. A low voltagetherapy circuit included in ICD 214 of FIG. 2 may include multiplepacing channels, including an atrial pacing channel, a right ventricularpacing channel, and a left ventricular pacing channel, to providesingle, dual or multi-chamber pacing in addition to the high voltagetherapy circuit used for delivering CV/DF shocks.

In some examples, ICD 14 (or ICD 214) is configured to monitor theimpedance of an electrode vector. For example, signal generator 84 mayapply a current drive signal to a pair of electrodes coupled to ICD 14.Sensing circuit 86 may detect the resulting voltage developed across thepair of electrodes. Impedance monitoring may be performed for detectinga lead or electrode issue and for selecting a therapy delivery electrodevector, a TCC transmitting electrode vector, or a sensing electrodevector based at least in part on the lead/electrode impedance. In otherexamples, ICD 14 or ICD 214 may be configured to monitor bioimpedance ina tissue volume, e.g., thoracic impedance or cardiac impedance, formonitoring a patient condition.

TCC transmitter 90 is configured to generate TCC signals fortransmission from a transmitting electrode vector selected from theelectrodes 24, 26, 28, 30 and housing 15 via a conductive tissuepathway. TCC transmitter 90 is configured to generate and transmit a TCCsignal, e.g., to communicate with pacemaker 100, sensor 50 or anotherIMD, or an external device 40. In some examples, signal generator 84includes switching circuitry for selectively coupling TCC transmitter 90to a selected transmitting electrode vector, e.g., using any two or moreof electrodes 24, 26, 28 30 and housing 15, e.g., housing 15 anddefibrillation electrode 24, for transmission of a TCC signal.

The TCC signal may be transmitted having a carrier signal with apeak-to-peak amplitude and carrier frequency selected to avoidstimulation of excitable tissue of patient 12. In some examples, thecarrier frequency of the TCC signal may be 100 kilohertz (kHz) orhigher. A TCC signal emitted or received, for example by electrode 24and housing 15, at a frequency of at least approximately 100 kHz may beless likely to stimulate nearby tissue, e.g., muscles or nerves, orcause pain than lower frequency waveforms. Consequently, a TCC signalhaving a frequency of at least approximately 100 kHz may have a higheramplitude than a lower frequency signal without causing extraneous nerveor muscle stimulation. A relatively higher amplitude signal may increasethe likelihood that pacemaker 100, pressure sensor 50 or anotherimplanted or external device, may receive the TCC signal from ICD 14 (orICD 214). The peak-to-peak amplitude of the TCC signal may be within arange from approximately 100 microamps to 10 milliamps (mA) or more,such as within a range from approximately 1 mA to approximately 10 mA.In some examples, the amplitude of the TCC signal may be approximately 3mA. A TCC signal having a frequency of at least approximately 100 kHzand an amplitude no greater than approximately 10 mA may be unlikely tostimulate nearby tissue, e.g., muscles or nerves, or cause pain. For atransmitting electrode vector having an impedance of 200 ohms injectinga current signal having an amplitude of 10 mA peak-to-peak, the voltagesignal at the transmitting electrode vector may be 2 Volts peak-to-peak.The voltage developed at the receiving electrode vector may be in therange of 0.1 to 100 millivolts peak-to-peak.

The modulation of the TCC signal may be, as examples, amplitudemodulation (AM), frequency modulation (FM), or digital modulation (DM),such as frequency-shift keying (FSK) or phase-shift keying (PSK). Insome examples, the modulation is FM toggling between approximately 150kHz and approximately 200 kHz. In some examples, the TCC signal has afrequency of 150/200 kHz and is modulated using FSK modulation at 12.5kbps. In the illustrative examples presented herein a TCC signal havinga carrier frequency of 100 kHz is modulated to encode data using binaryphase shift keying (BPSK). Balanced pulses of opposite polarity may beused to shift the phase of the TCC signal, e.g., by 180 degreespositively or negatively, and balance the charge injected into the bodytissue during the phase shift to minimize the likelihood of interferingwith cardiac event sensing operations of the cardiac event detector 85.Techniques for BPSK modulation of the TCC carrier signal using chargebalanced phase shifts are disclosed in U.S. patent application Ser. No.16/202,418 (Roberts, et al.) incorporated herein by reference in itsentirety. The data modulated on TCC signals, e.g., being sent topacemaker 100 or pressure sensor 50, may include “wakeup” commands,commands to deliver a therapy, and/or commands to collect or sendphysiological signal data, as examples.

The configuration of signal generator 84 including TCC transmitter 90illustrated in FIG. 5 may provide “one-way” or uni-directional TCC. Sucha configuration may be used if, for example, the ICD 14 is configured asa control device to transmit a command or request to another IMDconfigured as a responder, e.g., to pacemaker 100 or sensor 50, toprovide commands for pacing delivery or pressure signal acquisition, forinstance. In some examples, sensing circuit 86 may include a TCCreceiver 87 to facilitate “two-way” TCC between the ICD and another IMD.ICD 14 or ICD 214 may be configured to receive confirmation signals fromthe intended receiving, slave device to confirm that a transmitted TCCsignal was successfully received. In other examples, ICD 14 or ICD 214may receive commands via TCC receiver 87 from another IMD or externaldevice. The TCC receiver 87 may have more sensitivity than an RFtelemetry circuit 88, e.g., to compensate for lower signal-to-noiseratio signals from a transmitting device such as pacemaker 100 or sensor50. For instance, pacemaker 100 may generate relatively lowsignal-to-noise ratio signals by generating relatively small amplitudesignals due to its smaller power source, and/or to avoid stimulation ofadjacent cardiac tissue. A modulated or unmodulated carrier signal maybe received by TCC receiver 87 via electrodes selectively coupled tosensing circuit 86. TCC receiver 87 may include an amplifier, filter anddemodulator to pass the demodulated signal, e.g., as a stream of digitalvalues, to control circuit 80 for decoding of the received signal andfurther processing as needed.

In other examples, TCC receiver 87 and/or TCC transmitter 90 may bedistinct components separate from sensing circuit 86 and signalgenerator 84, respectively. For example, ICD 14 may include a TCCtransceiver that incorporates the circuitry of TCC receiver 87 and/orTCC transmitter 90. In this case, the functionality described withrespect to TCC receiver 87 and/or TCC transmitter 90 may be performedvia a distinct TCC component instead of being part of sensing circuit 86and signal generator 84.

Memory 82 may be configured to store a variety of operationalparameters, therapy parameters, sensed and detected data, and any otherinformation related to the monitoring, therapy and treatment of patient12. Memory 82 may store, for example, thresholds and parametersindicative of tachyarrhythmias and/or therapy parameter values that atleast partially define delivered anti-tachyarrhythmia shocks and pacingpulses. In some examples, memory 82 may also store communicationstransmitted to and/or received from pacemaker 100, pressure sensor 50 oranother device.

ICD 14 may have an RF telemetry circuit 88 including an antenna andtransceiver for RF telemetry communication with external device 40. RFtelemetry circuit 88 may include an oscillator and/or other circuitryconfigured to generate a carrier signal at the desired frequency. RFtelemetry circuit 88 further includes circuitry configured to modulatedata, e.g., stored physiological and/or therapy delivery data, on thecarrier signal. The modulation of RF telemetry signals may be, asexamples, AM, FM, or DM, such as FSK or PSK.

In some examples, RF telemetry circuit 88 is configured to modulate theTCC signal for transmission by TCC transmitter 90. Although RF telemetrycircuit 88 may be configured to modulate and/or demodulate both RFtelemetry signals and TCC signals within the same frequency band, e.g.,within a range from approximately 150 kHz to approximately 200 kHz, themodulation techniques for the two signals may be different. In otherexamples, TCC transmitter 90 includes a modulator for modulating the TCCsignal and/or TCC receiver 87 includes a demodulator for modulating theTCC signal rather than RF telemetry circuit 88.

FIG. 6 is a conceptual diagram of TCC transmitter 90 (or transmitterportion of a transceiver) according to one example. TCC transmitter 90may include a controller 91, drive signal circuit 92, polarity switchingcircuit 94, alternating current (AC) coupling capacitor 96, protectioncircuit 97 and voltage holding circuit 98. In other examples, TCCtransmitter 90 may include fewer or more components than the circuitsand components shown in FIG. 6. ICD power source 89 is shown coupled toTCC transmitter 90 to provide power necessary to generate TCC signals.While the controller 91, drive signal circuit 92, polarity switchingcircuit 94, AC coupling capacitor 96, protection circuit 97 and voltageholding circuit 98 are shown as discrete circuits by the blocks in FIG.6, it is recognized that these circuits may include common components ora common circuit may perform the functions attributed to the separatecircuit blocks shown in FIG. 6. For example, generating a carriercurrent signal having a carrier frequency and a peak-to-peak amplitudemay be performed by drive signal circuit 92 and polarity switchingcircuit 94 under the control of controller 91.

Controller 91 may include a processor, logic circuitry, data registers,a clock circuit and/or other circuitry or structures for providing thefunctionality attributed to controller 91 herein. Controller 91 mayinclude a dedicated clock circuit 93 for generating clock signals usedto control the frequency of the transmitted TCC signals. In otherexamples, controller 91 may be implemented within control circuit 80.The clock circuit 93 may be configured to provide a clock signal thatmay be used to transmit the TCC signal during a transmission sessionusing more than one frequency. For example, TCC transmitter 90 may beconfigured to provide a clock signal that may be used to transmit theTCC signal using at least three different frequencies, the TCC signalbeing modulated using FSK during a wakeup mode (e.g., modulating thesignal using two different frequencies) and switch to a datatransmission mode that includes transmitting data packets using acarrier signal at a third frequency (e.g., modulated using BPSK or othermodulation technique). For example, during a wakeup mode a beacon signalmay be transmitted using high and low alternating frequencies, which maybe centered on the frequency of the carrier signal. The beacon signalmay indicate the proximity or location of IMD 14 and/or its readiness tocommunicate. The beacon signal may be followed by a request to establisha communication, sometimes referred to as an “OPEN” request or command,transmitted at the carrier frequency. A clock signal generated by clockcircuit 93 may be required to enable generation of at least threedifferent frequencies of the TCC signal produced by drive signal circuit92 and polarity switching circuit 94 and passed to AC coupling capacitor96 in this particular example.

After switching from the wakeup mode to the data transmission mode,e.g., after receiving an acknowledgement signal from the other device,the TCC transmitter 90 may be configured to transmit subsequent TCCsignals at the carrier frequency, different than the distinct high andlow frequencies used during the beacon signal transmission. The carriersignal is modulated using BPSK in one example such that the TCC signalsare transmitted using a single frequency during the data transmissionmode.

The clock circuit 93 may operate at one clock frequency during thewakeup mode and at another clock frequency during the data transmissionmode. For example, clock circuit 93 may be controlled to operate at thelowest possible clock frequency that can be used to generate the highfrequency and low frequency cycles of the beacon signal during thewakeup mode to conserve power provided by power source 89. The clockcircuit 91 may be configured to operate at a higher frequency forcontrolling drive signal circuit 92 and polarity switching circuit 94 togenerate the carrier signal during signal transmission. The clockcircuit frequency may be changed between the wakeup and transmissionmodes under the control of controller 91 using digital trim codes storedin hardware registers.

TCC transmitter 90 is shown coupled to a transmitting electrode vector99 including defibrillation electrode 24 and housing 15 (of FIG. 1) inthis example. It is to be understood that TCC transmitter 90 may becoupled to one or more TCC transmitting electrode vectors selected fromany of the available electrodes coupled to the transmitting device asdescribed above via switching circuitry included in signal generator 84.Controller 91 may be configured to switchably connect a transmittingelectrode vector 99 to TCC transmitter 90 for transmission of TCCsignals, e.g., by controlling switches included in signal generator 84,which may be included in TCC transmitter 90 between AC couplingcapacitor 96 and transmitting electrode vector 99, e.g., in protectioncircuit 97. Controller 91 may select a transmitting electrode vectorfrom among multiple electrodes coupled to the transmitting device, whichmay include electrodes carried by the housing of the transmittingdevice, a transvenous lead, e.g., any of leads 204, 206 or 208 shown inFIG. 2, or a non-transvenous lead, e.g., extra-cardiovascular lead 16shown in FIG. 1.

Drive signal circuit 92 may include a voltage source and/or a currentsource powered by power source 89. In one example, drive signal circuit92 may be an active drive signal circuit generating a balanced,bi-directional drive current signal to balance the return current withthe drive current for a net zero DC current injected into the bodytissue via transmitting electrode vector 99. In another example, thedrive signal circuit 92 may include a charge pump and a holdingcapacitor that is charged by the charge pump to generate a currentsignal that is coupled to the transmitting electrode vector 99. In yetanother example, drive signal circuit 92 may include a current sourcethat is used to charge a holding capacitor included in drive signalcircuit 92.

The drive signal generated by drive signal circuit 92 may be a voltagesignal in some examples. In the illustrative examples presented herein,the drive signal circuit 92 generates a current signal to deliver TCCsignal current through the transmitting electrode vector 99 having adesired peak-to-peak amplitude, e.g., high enough to produce a voltagesignal on receiving electrodes of a receiving device that is detectableby the receiving device, which may be pacemaker 100, sensor 50 oranother intended receiving medical device, implanted or external. Thepeak-to-peak current amplitude is low enough to avoid or minimize thelikelihood of stimulation of tissue. A carrier signal that may begenerated by drive signal circuit 92 and polarity switching circuit 94may have a peak-to-peak amplitude in a range from approximately 1 mA toapproximately 10 mA, such as approximately 3 mA peak-to-peak, asdiscussed above. The voltage developed at the receiving electrode vectormay be in the range of 0.1 to 100 millivolts peak-to-peak.

Polarity switching circuit 94 receives the drive signal from drivesignal circuit 92 and includes circuitry configured to switch thepolarity of the drive signal current at a carrier frequency of the TCCsignal. For example, polarity switching circuit 94 may includetransistors and/or switches configured to switch the polarity of thedrive current signal at the frequency of the TCC signal. In someexamples, polarity switching circuit includes a respective one or moretransistors and/or switches coupled to each of electrode 24 and housing15, and the on-off states of the respective transistor(s) and/orswitch(es) are alternated to switch the polarity of the TCC signalcurrent between the electrodes at the carrier frequency. As discussedabove, the carrier frequency may be approximately 100 kHz. For example,the carrier frequency may be within a range from approximately 33 kHz toapproximately 250 kHz.

In some examples, RF telemetry module 86 may include a mixed signalintegrated circuit or other circuitry configured to provide a digitalversion of the modulated TCC signal to controller 91. In other examples,controller 91 is configured to produce the digital input signal formodulating the TCC carrier signal to encode communication data in thetransmitted signal. Controller 91 controls one or both of drive signalcircuit 92 and polarity switching circuit 94 to modulate the TCC carrierfrequency signal to generate the modulated TCC signal with an amplitude,phase shifts and/or frequency according to the encoding. For example,controller 91 may control polarity switching circuit 94 to toggle thefrequency of the carrier signal according to FSK modulation to encodethe communication data. In another example, controller 91 may controlpolarity switching circuit 94 to switch the polarity of the currentsignal after a desired portion of the carrier frequency cycle length toshift the phase of the AC current signal by 180 degrees according toBPSK modulation.

Polarity switching circuit 94 is capacitively coupled to thetransmitting electrode vector 99 (e.g., electrode 24 and housing 15 inthe example shown) via AC coupling capacitor 96. AC coupling capacitor96 couples the current signal output from polarity switching circuit 94to the transmitting electrode vector 99 to inject the current into theconductive body tissue pathway. AC coupling capacitor 96 may include oneor more capacitors coupled in series with one or each of the electrodesincluded in electrode vector 99. The AC coupling capacitor 96 is chargedto a DC operating voltage at the beginning of a TCC signal. AC couplingcapacitor 96 is selected to have a minimum capacitance that is based onthe frequency and the peak-to-peak current amplitude of the carriersignal being used to transmit beacon and data signals. As examples, ACcoupling capacitor 96 may have a capacitance of at least one nanofaradand up to ten microfarads for coupling a carrier signal having afrequency between 25 kHz and 250 kHz and peak-to-peak current amplitudeof 100 microamps to 10 milliamps. Larger capacitances may be used butmay increase the time required to charge the AC coupling capacitor to aDC operating voltage.

During a “cold start,” e.g., at the beginning of a TCC transmissionsession when AC coupling capacitor 96 is uncharged, the charging of ACcoupling capacitor 96 to the DC operating voltage may result in a lowfrequency current being injected into the body through the transmittingelectrode vector. This low frequency current is more likely to interferewith the operation of cardiac event detector 85 or otherelectrophysiological signal sensing circuits included in co-implantedIMDs or external devices coupled to the patient. Cardiac event detector85 and other electrophysiological signal sensing circuits of intended orunintended receiving devices may operate in a low frequency band, e.g.,1 to 100 Hz. As such, low frequency artifact at the start of TCC signaltransmission, during charging of the AC coupling capacitor 96, mayinterfere with cardiac event detector 85. After the DC operating voltageis established on AC coupling capacitor 96, the high frequency carriersignal, e.g., 100 kHz, is typically above the operating bandwidth ofcardiac event detector 85 and other electrophysiological sensingcircuitry of an IMD system and unlikely to cause interference or falseevent detection.

TCC transmitter 90 may include a voltage holding circuit 98 coupled toAC coupling capacitor 96. Voltage holding circuit 98 is configured tohold the AC coupling capacitor 96 at the DC operating voltage betweentransmitted TCC signals during a TCC transmission session and/or betweenTCC transmission sessions. By holding the AC coupling capacitor at a DCvoltage during time intervals between TCC signal transmissions,interference with sensing circuitry that may otherwise occur due to thelow frequency artifact injected during charging of the AC couplingcapacitor 96 to the DC operating voltage is minimized or avoided.

Examples of circuitry included in voltage holding circuit 98 aredescribed in U.S. Patent Application No. 62/591,806, incorporated hereinby reference in its entirety. In some examples, voltage holding circuit98 may include circuitry for floating AC coupling capacitor 96 at the DCvoltage between TCC signal transmissions. In other examples, voltageholding circuit 98 may include circuitry to actively hold the ACcoupling capacitor 96 at a DC voltage between TCC signal transmissions.A variety of circuitry may be conceived for preventing or minimizingdischarging of AC coupling capacitor 96 between TCC signaltransmissions. In this way, at the start of transmitting the next TCCsignal the AC coupling capacitor 96 is already at or near the DCoperating voltage. Without having to re-establish the DC voltage on theAC coupling capacitor 96, low frequency artifact injected into the TCCtissue pathway at the onset of the next TCC signal transmission isavoided or minimized. It is recognized that leakage currents may stillexist within TCC transmitter 90 and may cause some discharge of ACcoupling capacitor 96 between signal transmissions. Voltage holdingcircuit 98 may be used to minimize any discharge of AC couplingcapacitor 96 between transmitted TCC signals to minimize low frequencyinterference with sensing circuit 86 (FIG. 5) of the transmitting deviceas well as sensing circuits of other co-implanted IMDs and/or externaldevice coupled to the patient.

The TCC transmitter 90 may include protection circuit 97 that allows thedelivery of the TCC signal via electrodes coupled to other ICD circuitrybut protects the TCC transmitter 90 and other circuitry of the ICD 14from voltages that may develop across the electrodes, e.g., during aCV/DF shock delivered by therapy circuit 83 or an external defibrillatoras well as high voltages that may develop across the TCC transmittingelectrode vector during other situations such as an electrocauteryprocedure or magnetic resonance imaging. The circuitry within housing 15of ICD 14 protected by protection circuit 97 may include circuitry ofany of the components of ICD 14 illustrated in FIG. 5, such as controlcircuit 80, memory 82, sensing circuit 86, signal generator 84, and RFtelemetry circuit 88.

Protection circuit 97 may be coupled between drive signal circuit 92 andthe transmitting electrode vector 99, e.g., between AC couplingcapacitor 96 and electrode 24 and housing 15 as shown. In some examples,protection circuit 97 may include circuitry before and/or after ACcoupling capacitor 96. Protection circuit 97 may include, as examples,capacitors, inductors, switches, resistors, and/or diodes. Examples ofTCC signal generation and protection circuitry that may be utilized inconjunction with the signal transmission techniques disclosed herein aregenerally described in U.S. Pat. No. 9,636,511 (Carney, et al.),incorporated herein by reference in its entirety.

In some examples, TCC transmitter 90 may be controlled by controlcircuit 80 to transmit data via TCC multiple times throughout a cardiaccycle. In some cases, multiple transmissions at different times duringthe cardiac cycle increase the likelihood that the data is sent duringboth systole and diastole to make use of cardiac motion to increase thechance that the intended receiving electrode vector, such ashousing-based electrodes of pacemaker 100 or pressure sensor 50, isorientated in a non-orthogonal position relative to the transmittingelectrode vector. Multiple transmissions at different times during thecardiac cycle may thereby increase the likelihood that that the packetis received. While TCC transmitter 90 is shown coupled to a transmittingelectrode bipole (vector 99) in FIG. 6, it is to be understood thatmultiple transmitting electrode vectors may be coupled to TCCtransmitter 90 for transmitting a TCC current signal along multipleconductive tissue pathways for reception by multiple receiving electrodevectors or to increase the likelihood of being received by a singlereceiving electrode vector.

FIG. 7 is a conceptual diagram of a transmission session 300 that may beexecuted by transmitter 90 under the control of control circuit 80. Thechallenges of transmitting encoded information in a TCC signal includeavoiding unintentional electrical stimulation of nerve and muscletissue, including myocardial tissue, and avoiding or minimizinginterference with sensing circuitry included in one or more devices ofthe IMD system performing TCC while still successfully transmittinginformation in a time efficient and power efficient manner. Techniquesdisclosed herein include a method for transmitting TCC signals includingat least one ramped carrier signal to minimize low frequency artifact atthe beginning of a transmission session followed by transmitting awakeup signal (e.g., beacon signal) and encoded data packets having afrequency and amplitude that reduces the likelihood of stimulatingexcitable tissue.

Transmission session 300 may include a wakeup mode 310 followed by datatransmission mode 311 that may include transmission of one or more datapackets 330. In other instances, transmission session 300 does notinclude a wakeup mode. In the illustrative examples described herein, agroup of bits of encoded data is referred to as a data “packet.” In someuses, the term “packet” may imply that transmitted data is guaranteed tobe received along a communication pathway without error and aconfirmation signal indicating receipt without error may be returnedfrom the intended receiving device. In some applications, a group ofbits of encoded data may be referred to as a “datagram” whentransmission of the encoded data occurs without guarantee that the datareaches the intended receiver and without certainty that transmissionerrors did not occur. Groups of bits of encoded data 330 are referred toas “packets” herein, however, it is recognized that in some clinicalapplications the groups of bits 330 may be transmitted as datagrams,without guarantee that the receiving device actually received the dataerror-free.

Each transmission session 300 may, in some instances, begin with awakeup mode 310, as further described in conjunction with FIG. 8,followed by at least one data packet 330. Multiple data packets 330 maybe transmitted and assembled into a stream of data by the receivingdevice. In examples that include bi-directional communication, thetransmitting device may toggle between data transmission, during whichone data packet 330 is transmitted, and a receiving window 350 betweendata packets, during which the transmitting device waits for a responsefrom the intended receiver, e.g., a signal confirming receipt of thetransmitted packet, requested data sent back to the transmitter or otherrequested response to the received data packet. Examples of thestructure of each data packet 330 are described below, e.g., inconjunction with FIG. 11.

FIG. 8 is a diagram of one example of operations performed during thewakeup mode 310 by an IMD system, e.g., system 10 of FIG. 1 or system200 of FIG. 2, according to one example. Functions performed by thetransmitting device (TRN) are represented above the dashed line.Functions performed by the receiving device (RCV) are performed belowthe dashed line. In the example of ICD 14 (or ICD 214) being thetransmitting device, control circuit 80 controls TCC transmitter 90 totransmit an alternating current TCC ramp on signal 366 during wakeupmode 310, prior to the first beacon signal 312. Beacon signal 312 mayindicate to another device the proximity or location of IMD 14 and/orits readiness to enter into a communication session.

At the start of a transmission session, the early cycles of the carriersignal establish a DC voltage across the AC coupling capacitor 96.During this time, a low frequency current may be injected into the bodytissue conductive pathway via the TCC transmitting electrode vector. Thelow frequency current is more likely to cause interference with cardiacevent detector 85 of sensing circuit 86 (or other electrical signalsensing circuits of other implanted devices) than the relatively highcarrier frequency of the TCC signal. By starting each transmissionsession with a ramp on signal 366, the gradual charging of the ACcoupling capacitor 96 to the DC operating voltage is controlled in amanner that minimizes potential interference with cardiac event detector85 and/or other electrophysiological sensing circuits co-implanted IMDs.

The TCC ramp on signal 366 may be transmitted as an unmodulated carriersignal and may be up to 200 ms long in some examples. The TCC ramp onsignal 366 may be digitally controlled to step up the peak-to-peakamplitude of the AC carrier signal in a manner that minimizes any lowfrequency current that may be received at a sensing electrode vector toavoid interfering with electrical signal sensing. As described below inconjunction with FIG. 9, the TCC ramp on signal 366 may be stepped up inamplitude according to a step increment and step up interval thatresults in step changes in the voltage potential developed at a sensingelectrode vector that are below the sensitivity of the electrical signalsensing circuit 86. The duration of the ramp on signal 366 may beselected based on the time required to reach the peak-to-peak amplitudeof the carrier signal used to transmit the beacon signal 312 given aselected step increment and the step up interval. The maximum size ofthe step increment and minimum step up interval that can be used withoutcausing interference with electrophysiological signal sensing circuitryof the IMD system may depend on the programmed sensitivity ofelectrophysiological sensing circuity, e.g., cardiac event detector 85,the proximity of the transmitting electrode vector and the sensingelectrode vector and associated transimpedance and other factors thatinfluence the size of the DC voltage shift at a sensing electrodevector.

If the transmitting device includes a sensing circuit, such as sensingcircuit 86, the ramp on signal 366 may optionally be started during ablanking period 304 applied to the sensing circuit 86 following acardiac event 302. By starting ramp on signal 366 during a blankingperiod 304 applied to the sensing circuit 86 of the transmitting device,the DC voltage is established on the AC coupling capacitor 96 mostly orentirely during the blanking period 304 when the cardiac event detector85 is blanked and relatively immune to the low frequency artifact.

The blanking period 304 may be an automatic blanking period that thecontrol circuit 80 applies to the cardiac event detector 85 following anintrinsic or paced cardiac event 302. Cardiac event 302 may be anintrinsic cardiac event sensed by the cardiac event detector 85, andblanking period 304 may be a post-sense blanking period set in responseto detecting the intrinsic cardiac event, e.g., an R-wave or P-wave. Forexample, a post-sense blanking period may be applied to a senseamplifier or other cardiac event detection circuitry of sensing circuit86 in response to a cardiac event sensing threshold crossing. At othertimes, cardiac events 302 may be pacing pulses delivered at a pacinginterval 306, in which case blanking period 304 is a post-pace blankingperiod automatically applied to the sensing circuit 86 upon delivery ofthe pacing pulse by therapy circuit 83. A post-pace or post-shockblanking period may be applied to prevent saturation of the senseamplifier(s) of sensing circuit 86 during delivery of a pacing pulse orcardioversion/defibrillation shock. An automatic post-sense or post-paceblanking period may be in the range of 50 to 200 ms, for example 150 ms.

Control circuit 80 may alternatively apply a communication blankingperiod to cardiac event detector 85 that is independent of the timing ofcardiac electrical events, sensed or paced. In some cases, acommunication blanking period may be applied during the cardiac cyclebetween sensed or paced events. The communication blanking period may beapplied by control circuit 80 to the cardiac event detector 85 to enableTCC signal transmission to be initiated with ramp on signal 366 at anytime during the cardiac cycle, without waiting for an automaticpost-sense or post-pace blanking period.

A communication blanking period may be shorter or longer than theautomatic post-sense or post-pace blanking period. For example, acommunication blanking period may be in the range of 10 ms to 200 ms andmay depend on the programmed sensitivity of the cardiac event detector85. The communication blanking period may be applied for only a startingportion of ramp on signal 366, e.g., the first one or more step upincrements. The maximum duration of the communication blanking periodmay be limited based on the particular clinical application. Forexample, in the cardiac monitoring and therapy delivery IMD systems 10and 200 disclosed herein, the maximum time that cardiac event detector85 is blinded to detecting cardiac events may be 200 ms or less. Innon-cardiac applications, e.g., monitoring muscle or nerve signals,longer or shorter communication blanking intervals may be applied.

Starting ramp on signal 366 during a blanking period 304 may allowramping up of the carrier signal peak-to-peak amplitude more quickly inthat any low frequency artifact is not detected as a cardiac event bycardiac event detector 85 during a blanking period 304. Furthermore,during a post-sense, post-pace or post-shock blanking period, myocardialtissue is in a state of physiological refractoriness such that any lowfrequency signal injected at the beginning of a TCC signal startedduring a blanking period 304 is highly unlikely to capture themyocardial tissue.

In the example of the receiving device being pacemaker 100 havingsensing circuit 174, control circuit 170 may apply a post-sense orpost-pace blanking period in response to detecting an intrinsic cardiacevent or delivering a pacing pulse. The blanking period applied tosensing circuit 174 by control circuit 170 is applied to cardiac eventdetector 173 to prevent oversensing of non-cardiac events during theblanking period. The blanking period is not applied to TCC signaldetector 175, which may be operating in a polling mode including beaconsearch periods 320 and enabled to detect a beacon signal, even during ablanking period applied to cardiac event detector 173. Since bothpacemaker 100 and ICD 14 (or ICD 214) may be configured to sense cardiacelectrical signals from heart 8, and may be configured to detect pacingpulses delivered by another co-implanted device, both cardiac eventdetectors 85 and 173 of the transmitting and receiving devices,respectively, may be in a blanking period at the same time or at leastduring overlapping time periods. As such, by starting transmission of atleast the ramp on signal 366 of a new transmission session during ablanking period 304, sensing circuitry of other co-implanted devicesconfigured to detect cardiac electrical signals may also be in ablanking period, reducing the likelihood of low frequency interferencewith cardiac event detection by other sensing circuits during ACcoupling capacitor charging.

Depending on the duration of the TCC ramp on signal 366 and the blankingperiod 304, the first beacon signal 312 may be transmitted or at leaststarted during the blanking period 304. A DC operating voltage is atleast partially established on the AC coupling capacitor 96 during theTCC ramp on signal 366. In some examples, the AC coupling capacitor 96may continue to be charged to the DC operating voltage during the firstcycles of the beacon signal 312. In other instances, the AC couplingcapacitor 96 is fully charged to the DC operating voltage during ramp onsignal 366. While ramp on signal 366 is shown during a blanking period304, it is understood that ramp on signal 366 is provided to eliminateor minimize low frequency artifact injected into the TCC pathway duringcharging of the AC coupling capacitor 96 to the DC operation voltage. Assuch, the TCC transmission session beginning with ramp on signal 366 maybe started independent of the timing of blanking periods 304, with rampon signal 366 occurring at least partially or entirely outside ablanking period 304. Starting the TCC transmission session during ablanking period 304 is not required. By controlling the rate of chargingthe AC coupling capacitor 96 to the DC voltage during the ramp on signal366, low frequency artifact interference may be avoided.

One or more beacon signals 312 may be transmitted consecutively afterthe ramp on signal 366 to wake up the receiving device. In the exampleof pacemaker 100 being the receiving device, control circuit 170 maypower up TCC signal detector 175 (shown in FIG. 3B) periodically for abeacon search period 320 to detect the beacon signal 312. The beaconsignal 312 may be transmitted multiple times as needed until a responseis received from the receiving device. In the example shown, the beaconsignal 312 is sent four times, each time followed by a receiving period314 for waiting for acknowledgement signal 328 transmitted from thereceiving device to confirm detection of the beacon signal 312. Inresponse to receiving the acknowledgement signal 328 as indicated atarrow 316, the transmitter 90 stops transmitting the beacon signal 312and switches from the wakeup mode 310 to the transmission mode 311 asshown in FIG. 7.

A single ramp on signal 366 may be applied at the beginning of thewakeup mode 310, prior to the first beacon signal 312. Leakage currentthat may cause AC coupling capacitor 96 to discharge between beaconsignals 312 may be minimal, particularly when the periods 314 betweenbeacon signals 312 are relatively short. Discharge of the AC couplingcapacitor 96 due to leakage current between TCC signals may benegligible for up to one minute or even up to two minutes or more insome examples. The receiving periods 314 between beacon signals may beless than 10 seconds or even less than 1 second, such that negligibledischarge of the AC coupling capacitor 96 occurs between beacon signals312. In other examples, transmitter 90 may be configured to maintain theDC voltage established on the AC coupling capacitor 96 during the rampon signal 366 between beacon signals 312. For example, voltage holdingcircuit 98 may be controlled by controller 91 to float or actively holdAC coupling capacitor 96 at the DC voltage that was established duringthe ramp on signal 366 during the receiving period 314.

After the ramp on signal 366 and the first beacon signal 312, the ACcoupling capacitor 96 is already at (or near) the DC operating voltageat the start of the next beacon signal 312 such that low frequencyartifact during the early cycles of the next beacon signal is minimizedor avoided. Transmission of any subsequent beacon signals 312 does notrequire any additional ramp on signals immediately preceding eachadditional beacon signal, and the timing of the additional beaconsignals is not limited to the timing of cardiac events 302 and blankingperiods 304.

The beacon signal 312 may be shorter or longer than the ramp on signal366. In one example, the TCC ramp on signal 366 is up to 200 ms long andthe beacon signal 312 is up to 120 ms long. Beacon signal 312 mayinclude a single tone at the unmodulated carrier signal frequency, e.g.,100 kHz and may be transmitted for 100 ms, 200 ms, 500 ms, 1 second, 2seconds, or even up to 8 seconds. In other examples the beacon signal312 may vary between two or more tones within a range of the carriersignal. For instance, the beacon signal 312 may be an FSK signalmodulated between two different frequencies to transmit beacon signal312 having a pre-defined frequency signature that is detected by the TCCsignal detector.

The TCC signal detector of the receiving device, e.g., TCC signaldetector 175 included in pacemaker 100 or in pressure sensor 50, isconfigured to detect the beacon signal frequency and compare thefrequency to detection criteria. The TCC signal detector may include acomparator and counter configured to count pulses, e.g., by countingzero crossings, edges or other features of the voltage signal receivedat the receiving electrode vector, and comparing the count to a beacondetection threshold value. In other examples, the TCC signal detector ofthe receiving device may include a phase locked loop (PLL) that detectsthe frequency of the voltage signal at the receiving electrode vector.The frequency signal output of the PLL may be compared to the expectedbeacon signal frequency or frequency pattern.

As described below, during the TCC ramp on signal 366 the peak-to-peakamplitude of the carrier signal may be ramped up from a startingpeak-to-peak amplitude to the maximum peak-to-peak amplitude of thebeacon signal 312 using digitally controlled, charge balanced steps. Byslowly ramping the peak-to-peak amplitude of the carrier signal, therate of charging the AC coupling capacitor 96 to the DC operatingvoltage is controlled and any injected current signal during the ramp onsignal 366 is at a frequency that is substantially attenuated by afilter included in cardiac event detector 85 and/or produces anamplitude shift that is less than the sensitivity of the cardiac eventdetector 85. As used herein, the maximum peak-to-peak amplitude of asignal refers to the maximum peak-to-peak amplitude selected fortransmitting the carrier signal of a beacon signal 312 or a data packet330 and is not necessarily the maximum available peak-to-peak amplitudethat the transmitter 90 is capable of generating. The maximumpeak-to-peak amplitude of the carrier signal used to transmit a beaconsignal 312 may be greater than the maximum peak-to-peak amplitude of thecarrier signal during transmission of data packets 330. The greaterpeak-to-peak amplitude of the beacon signal 312 may increase thelikelihood of the beacon signal being detected by the receiving device.In other examples, the maximum peak-to-peak amplitude of the carriersignal transmitted as a beacon signal is the same as the maximumpeak-to-peak amplitude of the modulated carrier signal during datapacket transmission.

The receiving device controls the TCC signal detector, e.g., TCCdetector 175, to operate in a polling mode until a beacon signal 312 isdetected. The polling mode includes beacon search periods 320 scheduledat polling interval 322. Polling interval 322 may be a pre-determinedtime interval, e.g., from 0.5 seconds to 8 seconds. In other examples,the polling interval 322 may be variable, for example as generallydisclosed in the above-incorporated U.S. Pat. Application No. 62/591,810(Reinke, et al.).

The duration of the beacon search period 320 may be less than, equal toor greater than the beacon signal 312 in various examples. For instance,with no limitation intended, the beacon signal 312 may be approximately8 ms to 150 ms long. The beacon search period 320 may be 0.4 to 4 mslong. In other examples, the beacon signal 312 may be up to one secondlong, up to four seconds long, or even up to eight seconds long. Thebeacon signal transmission may be suspended if a therapy such as apacing pulse is scheduled for delivery, e.g., by therapy circuit 83.Transmission of a suspended beacon signal may be resumed after deliveryof the pacing pulse. The duration of the beacon search period 320 may beany portion of the duration of the beacon signal 312.

In FIG. 8, the first, earliest beacon search period 320 only partiallyoverlaps with the first beacon signal 312 of wakeup mode 310. If theoverlap of beacon search period 320 and beacon signal 312 is too short,beacon detection criteria applied by the receiving device may not bemet, and beacon signal 312 may go undetected. The first beacon searchperiod 320 is shown overlapping in time with the TCC ramp on signal 366.The TCC ramp on signal 366 may be undetected by the receiving device.The initial, low peak-to-peak amplitude of ramp on signal 366 may be toolow to be detected by the TCC signal detector. The ramp on signal 366may not meet the beacon signal detection criteria applied by thereceiving device. For example, the controller 91 may control the drivesignal circuit 92 and polarity switching circuit 94 to produce the TCCramp on signal 366 at the carrier signal frequency without modulation.The beacon signal 312 may be a modulated signal, e.g., using FSK or PSKmodulation of the carrier signal. The unmodulated frequency and phase ofthe carrier signal transmitted during the TCC ramp on signal 366 is notdetected as a beacon signal by the receiving device configured to detecta modulated beacon signal.

In other examples, beacon signal 312 may be transmitted as anunmodulated carrier signal having a carrier frequency and maximumpeak-to-peak amplitude approached or reached during the TCC ramp onsignal 366. In this case, the TCC signal detector 175 of the intendedreceiving device is configured to detect the unmodulated carrier signalto wake up and switch from the polling mode to the receiving mode. Asthe amplitude of the ramp on signal 366 rises, carrier signal cycles ofthe ramp on signal 366 may be detected by the TCC signal detector 175 ofthe receiving device during beacon search period 320. Beacon signaldetection may occur relatively early, e.g., within or even before thefirst beacon signal 312.

In the example shown, the first (leftmost) beacon signal 312 goesundetected. The second beacon search period 320 occurs during a laterbeacon signal 312. Beacon signal detection criteria may be reachedduring the beacon search period 320, e.g., a threshold number of carrierfrequency cycles, a threshold number of paired intervals of alternatingfrequencies of an FSK modulated beacon signal (as further describedbelow), or a threshold number and/or pattern of phase shifts of a BPSKmodulated beacon signal. The receiving device TCC signal detector 175may generate a beacon detection interrupt signal 324 that is passed tothe control circuit of the receiving device, e.g., control circuit 170.The control circuit may end the polling mode of the receiving device andswitch to a communication receiving mode to enable reception of datapackets 330 by the TCC signal detector.

In the example shown, the receiving device may include a TCC transmitterthat is controlled to transmit an acknowledgement signal 328 back to thetransmitting device to confirm beacon signal detection and that thereceiving device is waiting to receive data packet transmissions. Theacknowledgement signal 328 may be transmitted after a delay period 326to ensure that the transmitting device is no longer transmitting thebeacon signal 312 and has switched to a receiving period 314 and iscapable of receiving the acknowledgement signal. Acknowledgement signal328 may be the carrier signal transmitted for a predetermined timeinterval, e.g., 10 ms or less. The TCC signal transmission techniquesdisclosed herein which include a ramp on signal are described in thecontext of a controlling device transmitting beacon signals and datapackets to a responder (receiving device). However, it is to beunderstood that the TCC signal transmission techniques including a rampon signal may be used by the receiving device in generating andtransmitting the acknowledgment signal 328 as well.

During the receiving period 314, the transmitting device enables TCCreceiver 87 to detect the acknowledgement signal 328, e.g., by poweringthe TCC receiver 87 to enable the various filters, amplifiers,comparators, phase locked loops, or other circuitry to receive anddetect the acknowledgement signal 328. TCC receiver 87 may generate anacknowledgement detect signal 316 to control circuit 80. Control circuit80 switches the transmitting device from the wakeup mode 310 to a datatransmission mode 311 during which the data packets 330 are transmitted.

In the example shown, not all beacon signals 312 and the data packet 330are started during a blanking period 304. Using the TCC ramp on signaltechniques for charging the AC coupling capacitor disclosed herein, TCCsignals may be transmitted outside of the blanking periods 304, enablingTCC signal transmission independent of cardiac event timing. All or aportion of the first ramp on signal 366 of a TCC transmission sessionmay be started during a blanking period 304 to ensure that any small lowfrequency artifact is blanked. Subsequent TCC signals transmitted duringthe same TCC transmission session, such as multiple beacon signals 312and/or one or more data packets 330, may or may not start during ablanking period 304.

Beacon signal 312 may be transmitted multiple times during a cardiaccycle or over more than one cardiac cycle. In other examples, eachbeacon signal 312 may be followed by an OPEN command signal transmittedto the receiving device. The OPEN command signal may, for example, be arequest to initiate a TCC communication session and, in some instances,may include some communication parameters of the session. The receivingdevice may detect the beacon signal and switch to a data receiving mode.Upon receiving the subsequent OPEN command signal, the receiving devicemay transmit an acknowledgement signal back to the transmitting deviceto confirm to the transmitting device that the TCC signal detector ispowered on and ready to receive data transmissions.

FIG. 9 is a diagram of TCC ramp on signal 366, beacon signal 312, and aramp off signal 368 according to one example. During the ramp on signal366, controller 91 controls the drive signal circuit 92 to step up thepeak-to-peak amplitude of the carrier signal from a startingpeak-to-peak amplitude 370 (which may be starting from zero) to anending peak-to-peak amplitude 371. During the ramp off signal 368, thecontroller 91 may control the drive signal circuit 92 to step down thepeak-to-peak amplitude of the carrier signal from the maximumpeak-to-peak amplitude 372 of the beacon signal 312 to an endingpeak-to-peak amplitude 380, which may be zero.

Ramp on signal 366 may be the first signal transmitted during atransmission session, prior to the first beacon signal 312 as shown inFIG. 8. During ramp on signal 366, adjustments of the peak-to-peakamplitude may be controlled digitally by controller 91 to produce acharge-balanced ramped signal. In other examples, controller 91 maycontrol a large resistor having variable resistance to be graduallystepped down to control the rate that the peak-to-peak amplitude rampsup and the rate that the DC voltage is developed on the AC couplingcapacitor 96. The large resistor may be gradually stepped up to controlthe rate of discharge of the AC coupling capacitor 96 during ramp offsignal 368. The large resistor may be included in voltage holdingcircuit 98 to provide controlled discharge of AC coupling capacitor 96.In another example, the ramp on signal 366 is controlled digitally andthe ramp off signal 368 is a passive discharge of the AC couplingcapacitor 96 using a resistance included in voltage holding circuit 98that is selected to provide a long RC time constant producing a slowexponential discharge of the AC coupling capacitor 96 during the rampoff signal 368.

Controller 91 may control drive signal circuit 92 to step up thepeak-to-peak amplitude during ramp on signal 366 according to a stepincrement 374 made after each step up interval 376. The step increment374 and step up interval 376 are selected to minimize low frequencycurrent injected into the conductive tissue pathway, which may be resultin a low frequency voltage signal at a receiving electrode vectorintended to receive the subsequent beacon signal 312 or unintendedreceiving electrode vector(s) coupled to electrical signal sensingcircuitry included in the IMD system. The maximum peak-to-peak amplitude372 and frequency of the carrier signal of beacon signal 312 areexpected to be ineffective in stimulating cardiac or other muscle ornerve tissue. The ramp on signal 366, however, is provided to minimizeor avoid low frequency electrical interference in the IMD system at thebeginning of the TCC transmission session. By digitally controlling eachstep increment 374 and holding each stepped up peak-to-peak amplitudefor a predetermined number of carrier signal cycles (corresponding tostep up interval 376), the ramp on signal 366 is a charge balancedsignal.

The step increment 374 may be 1 microamp or less, e.g., 0.25 microampsas one example. The step up interval 376 may be 5 ms or less, e.g., 4ms. The step increment 374, step up interval 376 and total ramp onsignal duration 367 may be selected according to time constraints of aparticular application. For example, if the peak-to-peak amplitude ofthe ramp on signal 366 is to be adjusted from the starting peak-to-peakamplitude 370 to the ending peak-to-peak amplitude 371 within apost-sense blanking period of 150 ms, the step increment 374 and step upinterval 376 may be selected to minimize low frequency artifact (byminimizing each step size and maximizing each step increment) within thetime limitation of the blanking period while still reaching orapproaching the maximum peak-to-peak amplitude 372 of the carrier signalduring the beacon signal 312. Ending peak-to-peak amplitude 371 may beequal to or less than maximum peak-to-peak amplitude 372 of beaconsignal 312. For instance, ending peak-to-peak amplitude 371 may be onestep increment 374 less than maximum peak-to-peak amplitude 372.

The step increment 374, step up interval 376, and ramp on signalduration 367 are selected to minimize the amplitude and/or frequency oflow frequency artifact during charging of the AC coupling capacitor 96to the DC operating voltage within any given time constraints. A moregradual, longer ramp on signal 366 may be required depending on theprogrammed sensitivity and a high pass pole of the cardiac eventdetector 85 (or other sensing circuits included in the IMD system) andthe proximity of the receiving electrode vector to the transmittingelectrode vector. Higher sensitivity, lower high pass pole, and closerproximity may increase the likelihood of low frequency artifactinterfering with electrical signal sensing circuitry and may be avoidedby using a slower ramp rate (smaller step up interval 376 and/or longerramp on signal 366) and overall longer duration 367 of the ramp onsignal 366.

The duration 367 of ramp on signal 366 may be, without limitation, atleast 50 ms, at least 100 ms or at least 200 ms in some examples. Thepeak-to-peak current amplitude may be increased from 0 mA up to anending peak-to-peak amplitude 371 of 3 mA, 5 mA, 8 mA, or 10 mA asexamples. The frequency of any low frequency current signal injectedduring the ramp on signal 366, which may produce a voltage signal at areceiving electrode vector, may be limited to be less than 10 Hz, lessthan 5 Hz or even less than 2.5 Hz by selecting an appropriate stepincrement 366 and step up interval 376. In one example, a high pass poleof a filter included in cardiac event detector 85 is 5 Hz, and the rampon signal duration 367 is at least 100 ms during which the peak-to-peakcurrent amplitude is increased from 0 up to the maximum peak-to-peakamplitude 371 to limit any injected low frequency current signal to beless than 5 Hz.

In some cases, the DC operating voltage is established on the ACcoupling capacitor 96 within the ramp on signal 366. In other cases,charging of the AC coupling capacitor 96 to the DC operating voltage maybe ongoing at the expiration of the ramp on signal 366. The controlcircuit 80 of the transmitting device may monitor low frequency artifactat the sensing electrode vector, e.g., during ramp on signal 366 and/orat the start of beacon signal 312. Control circuit 80 may be configuredto signal the TCC transmitter controller 91 to adjust one or moreparameters controlling ramp on signal 366, e.g., step increment 374,step up interval 376, starting peak-to-peak amplitude 370, endingpeak-to-peak amplitude 371, ramp on signal duration 367, and/or thecarrier signal frequency to decrease the low frequency artifact ifdetected. In particular, if a cardiac event is sensed by cardiac eventdetector 85 during ramp on signal 366 or within a predetermined timeinterval after ramp on signal 366 (e.g., at the start of beacon signal312), one or more control parameters may be adjusted to minimize thelikelihood of low frequency artifact interfering with sensing circuit 86(and/or other electrical signal sensing circuit(s) included in the IMDsystem).

The step increment 374 and the step up interval 376 are shown as fixedvalues during ramp on signal 366. In other examples, the step increment374 and/or the step up interval 376 may be variable intervals. Forexample, if the beacon signal 312 starts during a post-sense, post-paceor communication blanking period but the ramp on signal duration 367extends later than the expiration of the blanking period, the stepincrement 374 may start at a first value and may be decreased to asecond value and/or the step up interval 376 may start at a shortervalue and be increased to a longer value in response to the blankingperiod ending. A lower ramp speed after the blanking period may reducethe likelihood of a low frequency current signal being injected into thebody tissue that causes interference with electrical signal sensingcircuitry included in the IMD system. In other examples, the stepincrement 374 may be a higher value at the start of ramp on signal 366and be gradually decreased during the ramp on signal 366 so that endingsteps occurring near the maximum peak-to-peak amplitude 371 are smallersteps than at the beginning of the ramp on signal 366. The step upinterval 376 may be adjusted one or more times during the ramp onsignal. For instance, the step up interval 376 may be shorter initiallyand longer near the end of the ramp on signal 366. Adjustments to thestep increment 374 and step up interval 376 may be made by drive signalcircuit 92 under the control of controller 91, for example, such thatmultiple settings of each of the step increment 374 and/or the step upinterval 376 are used during the ramp on signal 366.

The ramp off signal 368 may follow beacon signal 312 but does notnecessarily follow every (or any) beacon signal 312. The ramp off signal368 may be provided after a last beacon signal at the end of a wake upmode or only after a last data packet 330 at the end of a transmissionsession as further described below. If the ramp off signal 368 is atransmitted signal coupled to transmitting electrode vector 99, ramp offsignal 368 may be provided as a digitally controlled, charge balancedramp off signal to discharge the AC coupling capacitor 96 at a steppedrate that is slow enough to avoid or minimize low frequency artifactthat may interfere with electrical signal sensing circuits included inthe IMD system, including sensing circuit 86 of the transmitting device.Ramp off signal 368 is provided so that the DC voltage across the ACcoupling capacitor 96 is in a known state at the start of the next TCCsignal transmission. If the DC voltage established on the AC couplingcapacitor 96 during the ramp on signal 366 is held by voltage holdingcircuit 98 or not ramped down to control the discharge of the ACcoupling capacitor 96, the DC voltage across the AC coupling capacitoris likely to drift to an unknown voltage between transmitted signals orbetween transmission sessions due to leakage currents inherent in theIMD circuitry. If the voltage change is large enough when the next TCCsignal is transmitted, a large artifact may develop across a sensingelectrode vector, even if the TCC ramp on signal 366 is provided priorto the next TCC signal.

The ramp off signal 368 may be provided only after the last beaconsignal during the wake up mode in some examples. The time between beaconsignals 312 may be relatively short such that the AC coupling capacitorremains at the DC operating voltage between beacon signals. In somecases, the DC voltage established on the AC coupling capacitor duringthe ramp on signal 366 may be held between beacon signals 312 by voltageholding circuit 98. In these situations, the ramp off signal 368 is notprovided immediately and consecutively following every (or any) beaconsignal 312. In other examples, a ramp off signal 368 is not providedduring the wake up mode 310 at all and is only provided consecutivelyfollowing the last data packet 330 of the data transmission mode 311(shown in FIG. 7).

When ramp off signal 368 is provided, controller 91 may control thedrive signal circuit 92 and polarity switching circuit 94 to generatethe ramp off signal 368 as the AC carrier signal that steps down fromthe maximum peak-to-peak amplitude 372 of beacon signal 312 according toa step decrement 384 and a step down interval 386. The step decrement384 and step down interval 386 may or may not match the step increment374 and step up interval 376. In some examples, the rate of steppingdown the peak-to-peak amplitude is different than the rate of steppingup the peak-to-peak amplitude, which may result in different durations367 and 369 of ramp on signal 366 and ramp off signal 368, respectively.For instance, the step decrement 384 may have a larger absolute valuethan the step increment 374 and/or the step down interval 386 may beshorter than the step up interval 376.

As described above with regard to the ramp on signal control parameters,the ramp off signal control parameters, e.g., the step decrement 384and/or the step down interval 386, may be fixed or variable controlparameters which may increase or decrease between the beginning and endof the ramp off signal 368. The ramp off control parameters, e.g.,ending peak-to-peak amplitude 380, step decrement 384, step downinterval 386, and ramp off signal duration 369 may be adjustable toachieve ramping down of the current signal after beacon signal 312within a predetermined time interval and/or within maximum low frequencyartifact conditions as determined by monitoring a voltage signaldeveloped on a sensing electrode vector of the transmitting device.

In the example shown in FIG. 9, ramp off signal 368 may be a transmittedsignal that is the stepped down carrier signal produced by the drivesignal circuit 92 and polarity switching circuit 94 coupled to thetransmitting electrode vector 99 via AC coupling capacitor 96 togradually discharge the AC coupling capacitor. In other examples, theramp off signal 368 may be a non-transmitted signal. Controller 91 mayuncouple AC coupling capacitor 96 from the transmitting electrode vector99 at the end of the beacon signal 312 and couple the AC couplingcapacitor 96 to a resistor, which may be included in voltage holdingcircuit 98, to allow for a gradual discharge of the AC couplingcapacitor 96. In this case, ramp off signal 368 may be a continuous,e.g., exponentially decreasing, signal produced as the AC couplingcapacitor 96 is discharged through a resistor coupled to the AC couplingcapacitor rather than through a tissue pathway.

The beacon signal 312 shown in FIG. 9 is an FSK modulated beacon signalthat alternates between a high frequency 360 and a low frequency 362. ATCC signal detector configured to detect a single tone beacon signal,e.g., at the carrier signal frequency, may make false beacon signaldetections at an unacceptably high rate. EMI or other baseline noise maycause false beacon signal detections when no beacon signal is beingtransmitted. False wakeups unnecessarily use battery power of thereceiving device. In order to avoid false wakeups, transmitter 90 may becontrolled to transmit an FSK modulated beacon signal. An FSK modulatedbeacon signal may be discriminated from other noise or EMI that thereceiving device may be subjected to.

In the example of a 100 kHz carrier signal frequency, the high frequency360 may be in the range of 102 kHz to 120 kHz, and the low frequency 362may be in the range of 85 to 98 kHz. For example, the beacon signal 312may alternate between 98 kHz and 102 kHz, 95 kHz and 105 kHz, or 92 kHzand 108 kHz. Keeping the high and low frequencies within a bandpassfilter range of the carrier signal frequency may enable the TCC signaldetector 175 to use a common bandpass filter for detecting the FSKmodulated beacon signal and a PSK modulated carrier signal transmittedas a data packet or datagram. In another example, the FSK modulatedbeacon signal 312 alternates between a low frequency 362 of 85 kHz and ahigh frequency 360 of 115 kHz. The ramp on signal 366 may be transmittedas the unmodulated carrier signal with ramped amplitude prior to the FSKmodulation of the beacon signal 312. In other examples, FSK modulationusing the high and low frequencies 360 and 362 included in the beaconsignal 312 may be applied from the beginning of the ramp on signal 366.In this case, the FSK modulation may be detected early by the receivingdevice promoting an early transition from the wakeup mode 310 to thedata transmission mode 311.

The beacon signal 312 may include an end-of-beacon signature 364 in someexamples to enable positive detection of the end of the beacon signal312 by the receiving device and promote appropriate timing of thereceiving device waking up and powering up the TCC signal detector forreceiving TCC data transmissions. The relative orientation of thetransmitting and receiving electrode vectors may vary over a cardiaccycle and over a respiration cycle. As a result, the voltage signalamplitude at the receiving electrode vector may vary over time and mayeven drop out due to positional changes of the transmitting andreceiving electrode vectors caused by cardiac, respiratory or other bodymotion. To promote highly reliable positive wakeups, the beacon signal312 may be terminated with an end-of-beacon signature. The FSK beaconsignal 312 may be transmitted with a fixed number of cycles at each highand low frequency 360 and 362, e.g., 8 cycles, 12 cycles, 16 cycles, 24cycles, 32 cycles or more. In one example, the beacon signal istransmitted with 16 cycles of the high frequency 360 followed by 12cycles of the low frequency 362. The number of cycles transmitted ateach frequency may be selected so that each high and low frequency 360and 362 is transmitted for the same time interval to facilitatedetection of the FSK beacon signal and the end-of-beacon signature 364.

The end-of-beacon signature 364 may include any combination of highfrequency 360 and/or low frequency 362 intervals that is distinct fromthe beacon signal transmitted prior to the end-of-beacon signature 364.It is recognized that numerous variations of an end-of-beacon signaturemay be used that includes a distinct number of cycles of the highfrequency 360 and/or the low frequency 362 that is different than thenumber of cycles of each respective frequency delivered during the FSKmodulated beacon signal 312 leading up to the end-of-beacon signature364. In one example, each of the high frequency 360 and the lowfrequency 362 may be delivered for twice the number of cycles during theend-of-beacon signature 364 compared to prior to the end-of-beaconsignature. In the example given above including 16 cycles of highfrequency 360 alternating with 12 cycles of low frequency 362, theend-of-beacon signature may include 32 cycles of the high frequency 360alternating with 24 cycles of the low frequency 362.

While shown with only one pair of alternating high and low frequencies360 and 362 for the sake of illustration, it is to be understood thatthe end-of-beacon signature 364 may include multiple sequential pairs ofhigh and low frequency intervals, for example eight sequential pairs. Inthis example, using the high and low frequencies of 115 kHz and 85 kHzgiven above, the end-of-beacon signature 364 may be 56 ms in duration.Similarly, while only two pairs of alternating high and low frequencyintervals prior to the end-of-beacon signature 364 are shown for thesake of illustrating beacon signal 312 in FIG. 9, the number of pairedalternating high and low frequency intervals that precede theend-of-beacon signature 364 may be more than two and is selected toachieve a desired beacon signal duration.

For instance, the total duration of beacon signal 312 may be 50 ms to1000 ms after the ramp on signal 366 and before the ramp off signal 368.In some examples, ramp on signal 366 is up to 200 ms long, followed by abeacon signal 312 including FSK modulation of the carrier signal at themaximum peak-to-peak amplitude 372 for at least 8 ms and up to 120 ms ormore, and an end-of-beacon signature that may be 60 ms or less. If theramp off signal 368 is provided consecutively following beacon signal312, the ramp off signal 368 may be applied after the end-of-beaconsignature 364 so that the end-of-beacon signature 364 is transmitted atthe maximum peak-to-peak amplitude 372 to promote reliable detection ofthe beacon signal 312 by the receiving device. It is to be understoodthat the ramp off signal 368 at the end of the beacon signal 312 isomitted in some examples since the next TCC signal, which may be anotherbeacon signal or the first data packet during the data transmissionmode, is expected to occur relatively soon, e.g., within one minute orless.

FIG. 10 is a diagram of one example of a transmission session 400performed by a transmitting device of an IMD system, such as ICD 14 ofsystem 10 or ICD 214 of system 200, shown in FIG. 1 and FIG. 2respectively. The control circuit 80 controls transmitter 90 to starttransmission session 400 by signaling controller 91 to transition thetransmitter 90 from a sleep (minimized power) mode to the wakeup mode410. Controller 91 controls drive signal circuit 92 and polarityswitching circuit 94 to generate a ramped signal coupled to AC couplingcapacitor 96 during ramp on signal 466. The ramp on signal 466 may betransmitted as the carrier signal having a selected carrier frequencywith a starting peak-to-peak amplitude that is stepped up to an endingpeak-to-peak amplitude according to a step increment and step upinterval as described above. In some examples, the ramp on signal 466may include FSK modulated pairs of intervals of high and lowfrequencies. In other examples, the ramp on signal 266 is modulatedusing other modulation techniques. In yet other examples, the ramp onsignal 466 includes only the single-tone carrier signal.

The ramp on signal 466 is followed by a beacon signal 412. If a shorttime interval exists between ramp on signal 466 and the first beaconsignal 412, the AC coupling capacitor 96 may be held at the DC voltagedeveloped on the AC coupling capacitor during the ramp on signal 466during the time interval between ramp on signal 466 and the first beaconsignal 412. Any delay between ramp on signal 466 and the first beaconsignal 412 may be minimal such that any leakage current that might occurduring the delay results in minimal passive discharge of the AC couplingcapacitor 96. In some examples, ramp on signal 466 may be substantiallycontinuous with beacon signal 412 with no time gap in between.

The ramp on signal 466 includes a ramped carrier signal that reaches theending peak-to-peak amplitude as shown in FIG. 9, which may be themaximum peak-to-peak amplitude of the carrier signal used during beaconsignal transmission. The ramp on signal 466 may be up to 200 ms induration or more. The beacon signal 412 may be an FSK modulated signalincluding a predetermined number of pairs of alternating intervals ofhigh and low frequencies as described above, delivered at the beaconsignal maximum peak-to-peak amplitude. The beacon signal 412 may beterminated with an end-of beacon signature. If the ramp on signal 466 isdelivered at the carrier signal frequency and the beacon signal 412 isan FSK modulated signal, the TCC signal detector 175 of the receivingdevice (e.g., pacemaker 100 or pressure sensor 50) is configured todetect the beacon signal by searching for the expected alternating pairsof high and low frequency intervals. The carrier signal delivered duringthe ramp on signal 466 may not be recognized by the TCC signal detector175 as the start of the beacon signal since the carrier frequency isdifferent than the high and low frequencies of the FSK modulated signal.The beacon signal 412 is not terminated with a ramp off signal in thetransmission session 400.

The controller 91 may control the drive signal circuit 92 and polarityswitching circuit 94 to wait for a post-beacon interval 413 after beaconsignal 412 before transmitting an OPEN command 415. The post-beaconinterval 413 is provided to allow time for the receiving device todetect the beacon signal, including detecting the frequency pattern ofalternating intervals of high and low frequencies and the end-of-beaconsignature for an FSK modulated beacon, and power up the TCC signaldetector 175 to enable searching for the OPEN command 415. The beaconsignal 412 may be 50 ms to 1 second in duration and is 80 to 120 ms insome examples. The beacon signal 412 may be followed by a post-beaconinterval 413 that is 100 ms and 200 ms in duration. The voltage holdingcircuit 98 may hold the AC coupling capacitor 96 at the DC voltagedeveloped during the ramp on signal 466 during the post-beacon interval413. The OPEN command 415 may be 1 ms to 25 ms, e.g., 8 ms in duration.The OPEN command may be transmitted at the single-tone carrierfrequency, e.g., a 100 kHz signal, for a predetermined duration, e.g., 8ms at the beacon signal maximum peak-to-peak amplitude.

Relatively short beacon signals, e.g., 8 to 100 ms, may be repeated atmultiple times during the cardiac cycle to promote transmission at atime that the receiving electrode vector is parallel to the tissueconductance pathway of the injected current. In the example shown, eachbeacon signal 412 is followed by an OPEN command 415. In other examples,the beacon signal 412 may be transmitted repeatedly, e.g., two or moretimes during a cardiac cycle, separated by post-beacon intervals 413 anda single OPEN command 415 is transmitted after multiple short beaconsignals 412 to increase the likelihood of the beacon signal beingdetected by the receiving device in advance of the OPEN command 415.

The transmitting device control circuit 80 may enable the TCC receiver87 to search for an acknowledgement signal from the receiving deviceduring an acknowledgement receiving period 414 following each OPENcommand 415. Receiving period 414 may have a maximum duration forwaiting for an acknowledgment signal. If the acknowledgement signal isnot detected by the transmitting device by the expiration of thereceiving period 414, the transmitting device remains in the wakeup mode410 as indicated by the curved, dashed arrow. The beacon signal 412 maybe repeatedly delivered, followed by post-beacon intervals 413, OPENcommands 415 and receiving periods 414 until an acknowledgment detectsignal 416 is generated by the TCC receiver 87. In some cases, if apredetermined number of beacon signals 412 are delivered and theacknowledgment signal is not received, the controller 91 may controltransmitter 90 to wait for a beacon control interval 422 before sendinganother beacon signal 412. The voltage holding circuit 98 may be enabledto hold the AC coupling capacitor 96 at the DC voltage establishedduring ramp on signal 466 during the beacon control interval 422.

If beacon control interval 422 is relatively long, however, voltageholding circuit 98 may not be enabled to hold the AC coupling capacitor96 at the DC voltage during the beacon control interval 422. If thereceiving period 414 expires without detection of an acknowledgementsignal from the receiving device, controller 91 may control transmitter90 to provide a ramp off signal 468 after the last beacon signal 412 istransmitted and an acknowledgement signal is not received. Controller 91may control the drive signal circuit 91 to digitally step down thepeak-to-peak amplitude of a carrier signal from the maximum peak-to-peakamplitude of the beacon signals 412 according to a step decrement andstep down interval. In other examples, a variable resistor included involtage holding circuit 98 may be coupled to the AC coupling capacitor96 and the resistance may be gradually adjusted to control a slowdischarge rate of the AC coupling capacitor. In some examples, a largefixed resistor included in voltage holding circuit 98 is coupled to ACcoupling capacitor 96 during ramp off signal 468 to control the gradualdischarge of AC coupling capacitor 96. The ramp off signal 468 may occurduring the beacon control interval 422 and may not be a distinctlyseparate time interval. When a ramp off signal 468 is applied during thewake up mode 410, a ramp on signal 466 may be re-applied after beaconcontrol interval 422 to re-establish the DC voltage on the AC couplingcapacitor 96 prior to the next beacon signal transmission.

Upon detection of the acknowledgement signal transmitted from thereceiving device during the receiving period 414, an acknowledgementdetect signal 416 may be generated by the TCC receiver 87 and passed tocontrol circuit 80 of the transmitting device. Control circuit 80switches TCC transmitter 90 from the wakeup mode 410 to the datatransmission mode 411 to begin transmitting data packets 430. Thevoltage holding circuit 98 may hold the AC coupling capacitor 96 at theDC voltage established during the ramp on signal 466 to maintain the DCvoltage during the wakeup mode 410, between beacon signals 412 and fromthe time of the last OPEN command 415 until the first data packet 430.

In some examples a ramped charge adjustment period 452 may be appliedprior to the first packet 430. The ramped charge adjustment period 452may include a ramp on signal or a ramp off signal. The charge adjustmentperiod 452 may include a ramp on signal if the AC coupling capacitor 96is partially or wholly discharged during the time delay between the lastOPEN command 415 and the start of the first packet 430. For example, ifthe delay until the start of the first packet 430 is relatively long, ACcoupling capacitor 96 may at least partially discharge before the firstdata packet 430 due to leakage currents. During the charge adjustmentperiod 452, controller 91 may determine the remaining charge on the ACcoupling capacitor 96 to determine a starting amplitude of the ramp onsignal and control drive signal circuit 92 and polarity switchingcircuit 94 to pass the carrier signal to the AC coupling capacitor 96while stepping up the peak-to-peak amplitude of the carrier signal tothe maximum peak-to-peak amplitude of the data packets 430 according toa step increment and step up interval.

For example, controller 91 may include an analog-to-digital converter tosample the voltage signal across the AC coupling capacitor 96 at the endof the most recently transmitted TCC signal (e.g., the last beaconsignal 412 or OPEN command 415) to determine a target DC voltage. The ACcoupling capacitor voltage may be sampled during the charge adjustmentperiod 452 until the AC coupling capacitor 96 is adjusted to the targetvoltage. Since the AC coupling capacitor 96 may be at least partiallycharged, the ramp on signal applied during charge adjustment period 452may be shorter than the ramp on signal 466 applied at the “cold start”at the beginning of transmission session 400. In other examples thatinclude a ramp off signal 468 terminating the wakeup mode 410, a ramp onsignal 466 may be applied during charge adjustment period 452 at thestart of data transmission mode 411.

In some cases, the beacon signals 412 are transmitted at a higherpeak-to-peak amplitude than the data packets 430. In this case, thecharge adjustment period 452 may include a ramp down signal thatincludes an AC carrier signal having a current amplitude that is rampeddown from the maximum peak-to-peak amplitude of the beacon signals 412to the maximum peak-to-peak amplitude of the data packets 430.

Each data packet 430 is transmitted during the data transmission mode411 using a modulated carrier signal, e.g., an FSK or PSK modulatedsignal. Each data packet 430 may include a number of fields as describedbelow in conjunction with FIG. 11. In one example, controller 91 may beconfigured to control the drive signal circuit 92 and polarity switchingcircuit 94 to generate BPSK modulated signals during the datatransmission mode 411, e.g., by producing 180 degree phase shifts in thecarrier signal to encode digital signals in the modulated carrier.Methods for transmitting a BPSK signal by TCC transmitter 90 disclosedin the above-incorporated U.S. Patent Application No. 62/591,800(Roberts, et al.) and U.S. Patent Application No. 62/591,806 (Reinke, etal.) may be used in conjunction with the various examples of the ramp onsignal 466, ramp off signal 468 and charge adjustment period 452.

Packets 430 may be separated by receiving windows 450 during which TCCreceiver 87 may be enabled to detect signals transmitted by thereceiving device, such as a confirmation signal or other data requestedby the transmitting device. Voltage holding circuit 98 may be enabled tohold the AC coupling capacitor at the DC voltage originally establishedduring ramp on signal 466 during the receiving windows 450. In otherexamples if the time between packets 430 is relatively long, each datapacket 430 may be followed by a ramp off signal 468 and the next datapacket may be preceded by a ramp on signal 466 to re-establish the DCoperating voltage on the AC coupling capacitor 96 prior to transmissionof each packet 430.

The TCC transmission session 400 is terminated with a ramp off signal468 following the last packet 430. Controller 91 may control drivesignal circuit 92 to digitally step down the carrier signal amplitudefrom the maximum peak-to-peak amplitude of the data packets 430 tocomplete discharge of the AC coupling capacitor 96 during the ramp offsignal 468. The carrier signal amplitude may be stepped down accordingto a step decrement and step down interval as described above inconjunction with FIG. 9. In other examples, controller 91 may couple ACcoupling capacitor 96 to a discharge load, e.g., including a largeresistor included in voltage holding circuit 98, which may have anadjustable resistance, to slowly discharge the AC coupling capacitor 96according to the RC time constant of the discharge load and AC couplingcapacitor 96. Transmission session 400 may include a single ramp onsignal 466 consecutively followed by the first beacon signal 412 and asingle ramp off signal 468 consecutively following the last packet 430.

In other examples, one or more ramp on signals and one or more ramp offsignals may be applied to enable charging and discharging of the ACcoupling capacitor 96 to occur at controlled rates to minimize lowfrequency current being injected into the conductive body tissuepathway. The one or more ramp on signals 466 are each transmitted as ACsignals, which may be at the carrier frequency used to generate thebeacon signals 412 and data packets 430. The one or more ramp offsignals 468 may be transmitted AC signals at the carrier frequency or anon-transmitted, exponentially decaying signal produced by connectingthe AC coupling capacitor 96 to a high impedance resistive load with thedrive signal circuit 92 and polarity switching circuit 94 powered down.

FIG. 11 is a diagram of a data packet 430 that may be transmitted duringthe data transmission mode 411 by the transmitting device according toone example. Data packet 430 may include multiple fields 470, 472 and474 transmitted using a carrier signal and BPSK modulation. Asynchronization field 470 is transmitted as the first field at theunmodulated carrier signal frequency, e.g., 100 kHz, to provide acarrier lock for the demodulation by the TCC signal detector of thereceiving device. The synchronization field 470 may include apredetermined number of carrier frequency cycles or bits, e.g., witheight cycles per bit. The synchronization field 470 may be between 128and 256 cycles long in some examples. A ramp on signal 466 may beprovided prior to synchronization field 470 in some examples. Dependingon how often data packets 430 are being sent, a ramp on signal 466 maybe provided to charge the AC coupling capacitor to the DC voltage beforesynchronization field 470 of the data packet 430 is transmitted at thecarrier frequency and maximum peak-to-peak amplitude of the data packet.

In other examples, ramp on signal 466 is omitted at the start of datapacket 430. The leakage current may be negligible between TCC signaltransmissions within a transmission session such that AC couplingcapacitor 96 does not significantly discharge between the beaconsignal(s) and data packets. In some examples, voltage holding circuit 98may be enabled to hold the AC coupling capacitor at the DC voltageestablished during a preceding transmitted TCC signal, e.g., during theimmediately preceding beacon signal or preceding packet. The DC voltagemay be initially established during the first ramp on signal 466 appliedprior to beacon signal transmission during the wakeup mode 410 as shownin FIG. 10. The DC voltage initially established during a ramp on signal466 applied prior to the first beacon signal may be maintainedthroughout the transmission session by enabling voltage holding circuit98 to hold the AC coupling capacitor 98 at the DC voltage between TCCsignal transmissions such that ramp on signal 466 is not required at thebeginning of packet 430.

The carrier signal transmitted during the synchronization field 490 ismodulated during the preamble and data fields 492 and 494 according to abinary coded input signal, which may be produced by a modulator includedin controller 91. The preamble field 472 may follow the synchronizationfield 470 and may be encoded to communicate the type of packet beingtransmitted and the packet length, e.g., the number of data fields. Thepreamble field 472 may also include a key code to provide bit sampletiming to the receiving device, an acknowledgment request bit, sourceand/or destination address bits, or other bits or bytes that maynormally be included in a header or preamble field 472.

Data fields 474 include the information being communicated to thereceiving device, which may include commands to perform therapy deliveryor signal acquisition, requests for data, control parameter settings tobe used by the receiving device for sensing physiological signals and/ordelivering a therapy, or numerous other types of encoded informationthat enable coordination of the IMD system in monitoring the patient anddelivering therapy. Each data field 474 may include one byte and eachbyte may be a predetermined number of bits, e.g., 8 bits, 9 bits, 13bits or other predetermined number, representing a stream of digitalvalues. Each data packet 430 may include 1 to 256 data fields or bytes.In some examples, the data packet 430 may be terminated with a cyclicredundancy check (CRC) field to enable the receiving device to performan error check. While a particular example of a data packet 430 (ordatagram) is shown in FIG. 11, numerous data frame structures includingvarious fields may be conceived according to a particular clinicalapplication and IMD system that utilize ramp on and ramp off signals asdescribed herein.

If packet 430 is the last packet being transmitted in the transmissionsession, the last data field 474 may be followed consecutively by a rampoff signal 468. The ramp off signal 468 is provided to step down the DCvoltage charge of the AC coupling capacitor 96 in decrements that do notcause a detectable low frequency signal or DC voltage shift on a sensingelectrode vector coupled to an electrical signal sensing circuit of thetransmitting device or another co-implanted device, which may be theintended receiving device or an unintended receiver. Depending on howfrequently a data packet is transmitted, each data packet may bepreceded by a ramp on signal 466 and followed by a ramp off signal 468to minimize the likelihood of TCC signal interference with electricalsignal sensing circuitry of the IMD system. For example, if each datapacket 430 is transmitted at least once per second or even at least onceper minute, leakage current from the AC coupling capacitor may be smallenough to not require a ramp on signal 466. If leakage current isrelatively high and/or time between transmitted signals is relativelylong, controlled discharge of AC coupling capacitor 96 by applying rampoff signal 468 after and controlled recharging by applying ramp onsignal 466 before each data packet 430.

FIG. 12 is a conceptual diagram of a portion of a data field 474 thatmay be included in data packet 430 of FIG. 11, followed by ramp offsignal 468. Each data field 474 (or byte) of a packet 430 may betransmitted as the carrier signal 501 modulated using BPSK. The carriersignal 501 has a maximum peak-to-peak amplitude 572 and a carrierfrequency that is defined by the length of one carrier frequency cycle504. The carrier signal 501 has a positive polarity reaching half thepeak-to-peak amplitude 572 during one half of the carrier frequencycycle 504 and negative polarity reaching half the peak-to-peak amplitude572 during the other half of the carrier frequency cycle 504.

An input digital signal 520 may be generated by transmitter controller91 to control drive signal circuit 92 and/or polarity switching circuit94 of transmitter 90 (all shown in FIG. 6) to control modulation of thecarrier signal 501 during transmission of packet 430 ending with datafield 474 and followed by ramp off signal 468. Each bit 505-509 of thetransmitted data field 474 is transmitted with a predetermined number ofcarrier frequency cycles. Each bit value is encoded by controlling thephase shift between bits according to input digital signal 520. In theexample shown, each bit 505-509 includes eight carrier frequency cycles504. The bit value is encoded by controlling drive signal circuit 92and/or polarity switching circuit 94 to produce either a zero phaseshift between bits or a phase shift between bits. No phase shift maycorrespond to a digital “0” in the bit stream, and a phase shift maycorrespond to a digital “1” in the bit stream.

To illustrate, the first bit 505 may be a digital “0” and is followed bya phase shift 510 leading into the next bit 506. The phase shift is apositive 180 degrees in this example, but other phase shifts, between+360 degrees may be used. The TCC signal detector 175 of the receivingdevice that is locked into the frequency of the carrier signal 501 isconfigured to detect the phase shift 510. In response to detecting thephase shift 510, the TCC signal detector outputs a digital “1” indigital output signal 524 that is passed to the control circuit of thereceiving device for decoding. Bit 506 is followed by no phase shift 512according to the digital input signal 520 which changes from a digital“1” to a digital “0.” The TCC signal detector 175 of the receivingdevice detects no phase shift and produces a digital “0” for bit 507 inresponse to detecting no phase shift after the eight cycles of bit 506.

The next bit 507 is followed by a 180 degree positive phase shift 514 inaccordance with the change from a digital “0” to a digital “1” in theinput digital signal 520. The phase shift 514 is detected by the TCCsignal detector 175 and the bit value in the output digital signal 524changes from “0” to “1” for bit 508. The eight cycles of bit 508 arefollowed by no phase shift 516, in accordance with a change from “1” to“0” in the input signal 524. In response to no phase shift detection,the TCC signal detector 175 produces a digital “0” in the output digitalsignal 524 corresponding to the last bit 509. The last five bits of databyte 500 are represented in FIG. 12, however it is recognized that databyte 500 may include 2, 4, 8, 16, or other predetermined number of bits.The last four bits 506-509 may represent a nibble (four bits) of byte500 including at least 8 bits (an octet) in a hexadecimal encodingscheme. As can be seen in FIG. 12, the encoded data is transmittedduring a data packet by continuously transmitting the carrier signalwithout interruption while controlling the drive signal circuit 92 andthe polarity switching circuit 94 to shift the phase of the carriersignal.

As described above, each data packet 430 may include a header orpreamble that includes one or more bytes to provide various headerinformation including the number of data fields 474. As such, thereceiving device may be configured to ignore a ramp off signal 468 forthe purposes of producing digital output signal 524 after the expectednumber of data fields 474 has been received. The controller 91 maycontrol the drive signal circuit 92 and polarity switching circuit 94 tostep down the peak-to-peak amplitude of the carrier signal 501 duringthe ramp off signal 468 according to a step decrement 484 and step downinterval 486. In other examples, controller 91 may disable the drivesignal circuit 92 and polarity switching circuit 94 during the ramp offsignal 468. The AC coupling capacitor 96 may be coupled to a resistor involtage holding circuit 98 to gradually discharge, e.g., as shown by thecontinuous exponential decay signal 469. In some examples, each datapacket 430 is terminated with a ramp off signal 468. In other examples,only the last data packet 430 of a transmission session is terminatedwith a ramp off signal 468. As such, a single ramp off signal 468 mayoccur during a transmission session, such as transmission session 300 ofFIG. 7.

The ramp on signal 466 and ramp off signal 468 have been described inconjunction with an FSK modulated beacon signal and a BPSK modulateddata packet. It is to be understood that the ramp on signal 466 and rampoff signal 468 may be applied at least at the beginning and end,respectively, of a transmission session that utilizes other modulationtechniques and the ramp on and off signals 466 and 468 applied to thecarrier signal, modulated or unmodulated, are not limited to use with aparticular modulation scheme.

FIG. 13 is a flow chart 600 of a method for transmitting TCC signalsthat may be performed by an IMD system, e.g., system 10 of FIG. 1 orsystem 200 of FIG. 2, according to one example. The control circuit ofthe transmitting device, e.g., control circuit 80 of ICD 14 or ICD 214,determines that pending data are ready for TCC transmission at block601. As described above, the transmitting device may be ICD 14 or ICD214 operating as a controlling device and the receiving device may bepacemaker 100 or pressure sensor 50 operating as a responder. Thereceiving device may be a reduced function device with a smaller powersupply. For example, the receiving device may be configured to operatein a polling mode to be woken up by another device but may not beconfigured to operate in a wakeup mode to initiate TCC transmissionsessions. In other examples, the transmitting device performing themethod of flow chart 600 may be any IMD included in an IMD system thatis configured to initiate a transmission session, which may includepacemaker 100 or pressure sensor 50 in some examples.

Control circuit 80 powers up transmitter 90 at block 602 to switchtransmitter 90 to the wakeup mode from a sleep state, in which powersupplied to the circuitry of transmitter 90 is minimized. At block 604the ramp on signal is transmitted at the start of TCC signaltransmission. The ramp on signal may be transmitted via the transmittingelectrode vector 99 coupled to AC coupling capacitor 96 as theunmodulated, AC carrier signal having a ramped peak-to-peak amplitude.The AC ramp on signal is transmitted prior to transmission of the firstbeacon signal of the transmission session. In some examples, the ramp onsignal may be a modulated carrier signal, e.g., FSK modulation may beapplied to the ramp on signal corresponding to the FSK beacon signalmodulation as described in conjunction with FIG. 9. Performing FSKmodulation of the carrier signal during the ramp on signal may allowearly detection of the alternating frequencies of the beacon signal by areceiving device if the peak-to-peak amplitude of the TCC signal duringthe ramp on signal causes a detectable FSK modulated voltage signal tobe developed across the receiving electrode vector before the maximumpeak-to-peak amplitude of the beacon signal is reached.

The ramp on signal may be controlled according to a step increment, stepup interval, and total ramp duration as described above. In someexamples, control circuit 80 may be configured to control sensingcircuit 86 to monitor a voltage signal developed across a sensingelectrode vector at block 606. If the voltage signal meets artifactdetection criteria, the ramp control parameters may be adjusted at block608. For example, if the voltage signal crosses a threshold during theramp on signal, artifact detection criteria may be met. In someexamples, the ramp on signal is applied during a blanking or refractoryperiod applied to cardiac event detector 85 so that if the voltagesignal crosses a cardiac event detection threshold, it is ignored forthe purposes of determining a cardiac rate or detecting a cardiacrhythm. The voltage signal received by a sense amplifier or othercomponent of sensing circuit 86 from the sensing electrode vector duringthe ramp on signal may be monitored, however, for detecting a voltagechange induced by the TCC ramp on signal. The control circuit 80 maycontrol transmitter 90 to decrease the step increment and/or increasethe step up interval at block 608 to decrease any low frequency voltagesignal that may be occurring at a sensing electrode vector coupled tosensing circuit 86. In some examples, the adjustment at block 608 isapplied immediately during the ongoing ramp on signal. In otherexamples, the ramp on signal may be completed using the unadjustedcontrol parameters and the adjusted control parameters are appliedduring the next ramp on signal produced by transmitter 90, e.g., at thestart of the next transmission session.

After the ramp on signal, the beacon signal is transmitted at block 610,e.g., using FSK modulation of the carrier signal as described inconjunction with FIG. 9 and terminated with an end-of beacon signature.The beacon signal may be followed by an OPEN command as described abovein conjunction with FIG. 10. The transmitting device waits for anacknowledgement (ACK) signal from the receiving device at block 612. Ifthe acknowledgement signal is not received before the receiving windowtimes out, as described in conjunction with FIG. 10, the beacon signalmay be re-transmitted by returning to block 610. In the example of flowchart 600, a ramp on signal is applied a single time during thetransmission session, at the beginning of the wakeup mode. In otherexamples, if a ramp off signal is applied at the end of a beacon signal,the transmitter 90 may return to block 604 to apply another ramp onsignal before the beacon signal is transmitted again.

In response to receiving the acknowledgment signal at block 612, thecontroller 91 switches the operation of transmitter 90 from the wakeupmode to the data transmission mode at block 614. In other examples, thetransmitter 90 may transmit the beacon signal and switch to the datatransmission mode without detecting an acknowledgement signal. Thetransmitter 90 may switch to the data transmission mode after a delayinterval to allow the receiving device time to detect the beacon signaland switch from the polling mode to the receiving mode.

The transmitter 90 is controlled to transmit each data packet during thedata transmission mode, e.g., using BPSK modulation of the carriersignal, at block 616. Multiple data packets may be transmitted in asingle transmission session. While not shown explicitly in FIG. 13, itis understood that between data packets, the transmitting device mayenable a TCC receiver 87 for a receiving window, e.g., receiving window350 shown in FIG. 7, to detect and demodulate TCC signals requested fromand transmitted by the receiving device. In some cases, a return signalfrom the receiving device is not requested by the transmitting device sothat the receiving window may not be required.

After all pending data packets of the TCC transmission session are sent,as determined at block 618, the ramp off signal is produced at block620. The maximum peak-to-peak amplitude of the last data field may bestepped down according to a step decrement and step down interval toallow a controlled, gradual discharge the AC coupling capacitor 96through the transmitting electrode vector and tissue pathway. In someexamples, the voltage signal received at a sensing electrode vector maybe monitored by sensing circuit 86 to detect artifact during the rampoff signal. The ramp off signal control parameters may be adjusted bythe control circuit 80 in response to detecting a threshold level ofvoltage artifact by the sensing circuit 86. The step decrement may bedecreased and/or the step down interval increased to reduce the artifactthat may be caused during discharge of the AC coupling capacitor.

In other examples, the ramp off signal is produced at block 620 byuncoupling the AC coupling capacitor 96 from the transmitting electrodevector 99 and coupling the AC coupling capacitor 96 to a resistorincluded in voltage holding circuit 98. The AC coupling capacitor 96 isslowly discharged through the resistor according to the RC timeconstant. The controller 92 may disable the drive signal circuit 92 andthe polarity switching circuit 94 during the ramp off signal, e.g., bypowering down the drive signal and polarity switching circuits 92 and94.

The example of flow chart 600 includes a single ramp off signal appliedafter the last data field of the last data packet of the transmissionsession. In other examples, a ramp off signal may be applied at the endof each data packet, in which case a ramp on signal may be applied atthe beginning of each data packet. Monitoring a voltage signal at asensing electrode vector by sensing circuit 86 may be performed duringany one or more of the ramp on and/or ramp off signals applied during aTCC transmission session to allow adjustments of ramp control parametersas needed to minimize interference of the TCC signal with electricalsignal sensing circuits of the IMD system.

The transmission session is completed at block 622. Transmitter 90 maybe switched back to a low power, sleep state at block 622, until thenext pending TCC transmission session. In some instances, if the nextTCC transmission session is expected to occur relatively soon, e.g.,within up to one minute or within up to ten minutes as examples, theramp off signal is not applied at block 620. The next transmissionsession may be started without applying a ramp on signal. The leakagecurrents may be minimal such that minimal discharge of the AC couplingcapacitor 96 occurs between transmission sessions. The voltage holdingcircuit 98 may be enabled by controller 91 to hold the AC couplingcapacitor 96 at the DC voltage established during the most recentlycompleted transmission session until the next transmission session. Assuch, not all transmission sessions may be required to include a ramp onsignal and a ramp off signal.

FIG. 14 is a conceptual diagram of a method for transmitting TCC signalsduring multiple transmission sessions 700 a-n according to one example.If multiple transmission sessions 700 a-n are expected to be scheduledto occur sequentially within a relatively short period of time, e.g.,within several minutes or within one hour of each other, control circuit80 may control transmitter 90 to apply a ramp on signal 766 only at thebeginning of the first transmission session 700 a and apply the ramp offsignal 768 only at the end of the last transmission session 700 n. Eachtransmission session 700 a-n may include a wakeup mode 710 during whichat least one beacon signal is transmitted and a data transmission mode711 during which at least one data packet or datagram is transmitted.Voltage holding circuit 98 may optionally be controlled to hold the DCvoltage initially established (at least partially) during the ramp onsignal 766 in between beacon signals and data packets within atransmission session and during time intervals 720 between consecutivetransmission sessions 700 a-n. When the intervals 720 between successivetransmission sessions 700 a-n are relatively short, e.g., 10 minutes orless, 5 minutes or less, 2 minutes or 1 minute or less, a ramp on signal766 may be transmitted only at the start of the first transmissionsession 700 a, and a ramp off signal 768 may be produced only at the endof the last transmission session 700 n. When relatively long intervals720 are expected between transmission sessions, e.g., more than tenminutes or more than one hour, a ramp on signal and a ramp off signalmay initiate and terminate each transmission session 700 a-n.

Thus, various examples of a method and apparatus for TCC performed by amedical device system have been presented in the foregoing descriptionwith reference to specific embodiments. It is appreciated that variousmodifications to the referenced embodiments, including combining variousaspects of the TCC signal transmission and detection methods indifferent combinations than the specific combinations described here,may be made without departing from the scope of the disclosure and thefollowing claims. It should be understood that, depending on theexample, certain acts or events of any of the methods described hereincan be performed in a different sequence, may be added, merged, or leftout altogether (e.g., not all described acts or events are necessary forthe practice of the method). Moreover, in certain examples, acts orevents may be performed concurrently, e.g., through multi-threadedprocessing, interrupt processing, or multiple processors, rather thansequentially. In addition, while certain aspects of this disclosure aredescribed as being performed by a single circuit or component forpurposes of clarity, it should be understood that the techniques of thisdisclosure may be performed by a combination of circuits or componentsassociated with, for example, a medical device and/or a single circuitor component may perform multiple functions that are represented asseparate circuits or components in the accompanying drawings.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored as one or more instructions orcode on a computer-readable medium and executed by a hardware-basedprocessing unit. Computer-readable media may include computer-readablestorage media, which corresponds to a tangible medium such as datastorage media (e.g., RAM, ROM, EEPROM, flash memory, or any othernon-transitory computer-readable medium that can be used to storedesired program code in the form of instructions or data structures andthat can be accessed by a computer).

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

Thus, a system has been presented in the foregoing description withreference to specific examples. It is to be understood that variousaspects disclosed herein may be combined in different combinations thanthe specific combinations presented in the accompanying drawings. It isappreciated that various modifications to the referenced examples may bemade without departing from the scope of the disclosure and thefollowing claims.

What is claimed is:
 1. A device comprising: a housing; and a tissueconductance communication (TCC) transmitter enclosed by the housing andincluding a coupling capacitor for coupling TCC signals to atransmitting electrode vector, the TCC transmitter configured to:generate a TCC ramp on signal having a peak-to-peak amplitude that isstepped up from a starting peak-to-peak amplitude to an endingpeak-to-peak amplitude according to a step increment and a step upinterval; transmit the TCC ramp on signal via the coupling capacitorcoupled to the transmitting electrode vector; and transmit a second TCCsignal after the TCC ramp on signal, the second TCC signal having afirst maximum peak-to-peak amplitude that is equal to or greater thanthe ending peak-to-peak amplitude of the TCC ramp on signal.
 2. Thedevice of claim 1, wherein the TCC transmitter is configured to:subsequent to transmitting the second TCC signal, generate at least oneTCC data signal comprising a modulated carrier signal encoded with datausing a first modulation scheme and having a second maximum peakamplitude; and transmit the at least one TCC data signal.
 3. The deviceof claim 2, wherein the TCC transmitter is configured to modulate thesecond TCC signal using a second modulation scheme different than thefirst modulation scheme.
 4. The device of claim 2, wherein the TCCtransmitter is further configured to: generate a TCC ramp off signalafter the at least one TCC data signal, the TCC ramp off signal having adecreasing amplitude starting from the second maximum peak-to-peakamplitude and stepped down from the second maximum peak-to-peakamplitude according to a step decrement and a step down interval; andtransmit the TCC ramp off signal.
 5. The device of claim 2, wherein theTCC transmitter is configured to produce the ramp off signal by:disabling a drive signal; coupling the coupling capacitor to a resistor;and allowing the coupling capacitor to discharge through the resistor.6. The device of claim 2, wherein: the TCC transmitter further comprisesa voltage holding circuit: the TCC transmitter is further configured tocontrol the voltage holding circuit to hold the coupling capacitor at anoperating voltage for a time interval between the second TCC signal andthe at least one TCC data signal, the operating voltage being at leastpartially established on the coupling capacitor during the TCC ramp onsignal.
 7. The device of claim 2, wherein the TCC transmitter isconfigured to produce a ramp adjustment signal between the second TCCsignal and the at least one TCC data signal to adjust a charge on thecoupling capacitor.
 8. The device of claim 2, wherein the TCCtransmitter is configured to: generate the second TCC signal having thefirst maximum peak-to-peak amplitude greater than the second maximumpeak-to-peak amplitude of the at least one TCC data signal; and generatea ramp adjustment signal between the second TCC signal and the at leastone TCC data signal, the ramp adjustment signal having an amplitude thatdecreases from the first maximum peak-to-peak amplitude to the secondmaximum peak-to-peak amplitude.
 9. The device of claim 2, wherein theTCC transmitter is configured to: generate the second TCC signal havingthe first maximum peak-to-peak amplitude greater than the second maximumpeak-to-peak amplitude of the at least one TCC data signal; produce aTCC ramp down signal after the second TCC signal having an amplitudethat decreases from the first maximum peak-to-peak amplitude to a zeroamplitude; and produce a second TCC ramp on signal between the secondTCC signal and the at least one TCC data signal, the second TCC ramp onsignal having a peak-to-peak amplitude that increases to the secondmaximum peak-to-peak amplitude.
 10. The device of claim 1, wherein theTCC transmitter is configured to: generate the first TCC ramp on signalat a carrier frequency; and generate the second TCC signal as afrequency shift keying modulated signal having alternating timeintervals of a high frequency greater than the carrier frequency and alow frequency less than the carrier frequency.
 11. The device of claim1, wherein TCC transmitter is configured to: terminate a first TCCtransmission session comprising at least the TCC ramp on signal and thesecond TCC signal; and start a second TCC transmission session without aTCC ramp on signal, the second TCC transmission session comprising aplurality of TCC signals including at least one beacon signal.
 12. Thedevice of claim 1, wherein the TCC transmitter is configured to:terminate a first TCC transmission session comprising a plurality oftransmitted TCC signals including at least the TCC ramp on signal andthe second TCC signal by producing a TCC ramp off signal after a lastone of the plurality of transmitted TCC signals, the TCC ramp off signalhaving a decreasing amplitude starting from a peak amplitude of theplurality of transmitted TCC signals; and start a second TCCtransmission session by generating a second TCC ramp on signal.
 13. Thedevice of claim 1, further comprising: a sensing circuit configured toreceive a cardiac electrical signal via a plurality of electrodes; and acontrol circuit coupled to the sensing circuit and the TCC transmitter,the control circuit configured to: identify a cardiac event within thecardiac electrical signal, apply a blanking period to the sensingcircuit in response to identifying the cardiac event; and control theTCC transmitter to start transmitting the TCC ramp on signal during theblanking period.
 14. The device of claim 1, wherein the TCC transmitteris configured to generate the TCC ramp on signal having alternating timeintervals of a first frequency and a second frequency less than thefirst frequency.
 15. The device of claim 1, further comprising: asensing circuit configured to receive a cardiac electrical signal via aplurality of electrodes; and a control circuit coupled to sensingcircuit and the TCC transmitter, the control circuit configured to:monitor a voltage signal received by the sensing circuit; compare thevoltage signal to artifact detection criteria; and control the TCCtransmitter to adjust a parameter of the TCC ramp on signal in responseto artifact detection criteria being met.
 16. A method comprising:generating, with a tissue conduction communication (TCC) transmitter, aTCC ramp on signal having a peak-to-peak amplitude that is stepped upfrom a starting peak-to-peak amplitude to an ending peak-to-peakamplitude according to a step increment and a step up interval;transmitting, with the TCC transmitter, the TCC ramp on signal via acoupling capacitor coupled to a transmitting electrode vector; andtransmitting, with the TCC transmitter, a second TCC signal after theTCC ramp on signal, wherein transmitting the second TCC signal comprisestransmitting the second TCC signal having a first maximum peak-to-peakamplitude that is equal to or greater than the ending peak-to-peakamplitude of the first TCC ramp on signal.
 17. The method of claim 16,further comprising: subsequent to transmitting the second TCC signal,generating at least one TCC data signal comprising a modulated carriersignal encoded with data using a first modulation scheme and having asecond maximum peak-to-peak amplitude; and transmitting the at least oneTCC data signal.
 18. The method of claim 17, further comprisingmodulating the second TCC signal using a second modulation schemedifferent than the first modulation scheme.
 19. The method of claim 17,further comprising: generating a TCC ramp off signal after the at leastone TCC data signal, the TCC ramp off signal having a decreasingamplitude starting from the second maximum peak-to-peak amplitude andstepped down from the second maximum peak-to-peak amplitude according toa step decrement and a step down interval; and transmitting the TCC rampoff signal.
 20. The method of claim 19, wherein generating the TCC rampoff signal comprises: disabling a drive signal; coupling the couplingcapacitor to a resistor; and allowing the coupling capacitor todischarge through the resistor.
 21. The method of claim 17, furthercomprising: holding the coupling capacitor at an operating voltage for atime interval between the second TCC signal and the at least one TCCdata signal, the operating voltage being at least partially establishedon the coupling capacitor during the first AC ramp on signal.
 22. Themethod of claim 17, further comprising producing a ramp adjustmentsignal between the second TCC signal and the at least one TCC datasignal to adjust a charge on the coupling capacitor.
 23. The method ofclaim 17, further comprising: generating the second TCC signal with thefirst maximum peak-to-peak amplitude greater than the second maximumpeak-to-peak amplitude of the at least one TCC data signal; andproducing a ramp adjustment signal between the second TCC signal and theat least one data signal, the ramp adjustment signal having an amplitudethat decreases from the first maximum peak-to-peak amplitude to thesecond maximum peak-to-peak amplitude.
 24. The method of claim 17,further comprising: generating the second TCC signal with the firstmaximum peak-to-peak amplitude greater than the second maximumpeak-to-peak amplitude of the at least one TCC data signal; producing aTCC ramp down signal after the beacon signal having an amplitude thatdecreases from the first maximum peak-to-peak amplitude to a zeroamplitude; and producing a second TCC ramp on signal between the atleast one beacon signal and the at least one TCC data signal, the secondTCC ramp on signal having a peak-to-peak amplitude that increases to thesecond maximum peak-to-peak amplitude.
 25. The method of claim 16,further comprising: generating the first TCC ramp on signal at a carrierfrequency; and generating the second TCC signal as a frequency shiftkeying modulated signal having alternating time intervals of a highfrequency greater than the carrier frequency and a low frequency lessthan the carrier frequency.
 26. The method of claim 16, furthercomprising: terminating a first TCC transmission session comprising atleast the TCC ramp on signal and the second TCC signal; and starting asecond TCC transmission session without a ramp on signal, the second TCCtransmission session comprising a plurality of TCC signals including atleast one beacon signal.
 27. The method of claim 16, further comprising:terminating a first TCC transmission session comprising a plurality oftransmitted TCC signals including at least the TCC ramp on signal andthe second TCC signal by producing a ramp off signal after a last one ofthe plurality of transmitted TCC signals, the ramp off signal having adecreasing amplitude starting from a peak amplitude of the plurality oftransmitted TCC signals; and starting a second TCC transmission sessionby generating a second TCC ramp on signal.
 28. The method of claim 16,further comprising: identifying a cardiac event, applying a blankingperiod to a sensing circuit in response to identifying the cardiacevent, the sensing circuit configured to receive a cardiac electricalsignal obtained via a plurality of electrodes; and transmitting the TCCramp on signal during the blanking period.
 29. The method of claim 16,further comprising generating the TCC ramp on signal as a frequencymodulated signal having alternating time intervals of a first frequencyand a second frequency less than the first frequency.